Documentation: brand new layout for NuttX documentation - #20413

I'm opening this PR because I strongly believe NuttX deserves a better and more welcoming documentation, one that will pull new joiners in instead of pushing them away.

I have spent a couple of weeks reviewing it and I came to the conclusion that it would be impossible to do this in small changes across several commits.

I really tried to only reorganize the pages we already have, but that proved impossible as well. So 10 pages had to be introduced, otherwise this new "outfit" would have been incomplete, and the reviewers would have complained
(rightly so). If this one is accepted, the second PR intends to fix the issues in the documentation itself. But that is a topic for the next PR (and in this case it is totally possible to fix page by page, commit by commit).

Finally, yes, I have used AI to support this documentation refactor, otherwise it would have been pretty much impossible (pretty-much != it would be). The good thing is that I have audited it as well, via scripts. That is why the information below is a little bit big, but from my perspective it is quite important to explain everything that is being proposed here.

In parallel, I'm also opening a discussion topic on email-list / discord so we can talk better about it. Hopefully this patch achieve its purpose :)

Note: this replaces #20394, which I closed myself. The old PR stays
open to read, including the review that led to this one.

Thanks!

Summary

Why.
NuttX is a small, minimalist RTOS. The documentation is not small and
not organised, and that gap is what this PR fixes.

What this does.
Every page now sits under the subsystem it describes, so the sidebar follows the source tree instead of the order things were written. The top level drops from 19 entries to 9, and 520 redirects keep old links alive.

What did NOT change.
The text of the pages. Of the 1577 .rst pages, 1567 keep the
words that are already in master. Only their address, their links and
their place in the tree changed.

What text DID change.
Ten pages. Nine are the landing page of a chapter, which has to exist
for the new structure. The tenth is the only one with technical
content: libs/libbuiltin/ had no page at all.

index                  the front page        os/time/index       Time and timers
os/index               OS Design             about/index         About
os/scheduling/index    Scheduling            developing/index    Developing NuttX
os/interrupts/index    Interrupts            ReleaseNotes/index  Release notes
os/ipc/index           IPC                   os/libs/libbuiltin  libs/libbuiltin/

What else changed, and why.
When I said "nothing has changed" was related to the documentation only.
Apart of the reorganization, even non-page files did change, and here is
each one:

Five SVG diagrams come with the ten pages above. They are hand-written
XML: no editor metadata, no scripts, no external references.

Table of contents, before and after.
As requested:

CURRENT (19 top-level)            PROPOSED (9 top-level)
  Introduction                      Introduction
  Getting Started                   Getting Started
  Contributing                      Supported Platforms
  Inviolables                       Guides
  Supported Platforms               OS Design          <- new
  OS Components                     API Reference
  Applications                      Applications
  Implementation Details            Developing NuttX   <- new
  API Reference                     About              <- new
  FAQ
  Debugging
  Testing
  Guides
  Standards
  Security
  Glossary
  Logos
  Tags Index

Nothing was deleted. Every old chapter is still there, one level down:

Contributing, OS Components, Implementation, Testing  ->  Developing NuttX
FAQ, Security, Glossary, Logos, Release Notes         ->  About
Debugging                                             ->  Guides
Standards                                             ->  Introduction
Inviolables                                           ->  Introduction
Tags Index                                            ->  Supported Platforms

Impact

Is new feature added? NO
Is existing feature changed? NO (documentation only)

• Users: the pages that moved get a redirect from their old address,
  so existing links and bookmarks keep working, including links that jump
  to a section. Most pages did not move at all.
• Build: no change to the OS build. Documentation/Pipfile gains one
  dependency, sphinx-reredirects, pinned in the lock file.
• Hardware: none. Nothing outside Documentation/ is touched.
• Documentation: this is the change.
• Security: none.
• Compatibility: no source, header or Kconfig symbol is touched.

Documentation-only change, so it is tested with make html as
CONTRIBUTING says, plus a check script of my own.

Host: Linux x86_64 (Ubuntu 26.04), Python 3.12.7, Sphinx 6.2.1.
Board: none applicable, no code is touched.

$ cd Documentation && make html
build succeeded.
0 warnings under -W, 0 documents outside a toctree

$ python3 validate_reorg.py e8a3d4ee1c
-- 2. .rst PAGES IN HEAD (1581) ------------------------------------------
    never touched                            791
    changed by the move only                 467
    moved, byte for byte identical           310
    new or rewritten text (the 10)            10
    halves of one split page (section 3)       2
    new, no text (toctree only)                1
    TOTAL                                   1581
-- 6. PAGES DELETED FOR GOOD (1) -----------------------------------------
    components/drivers/character/leds/index.rst            0 sentences, 0 nowhere now
-- 7. REDIRECTS (495 rules) ----------------------------------------------
    pages no longer at their old path            384
    rules for those pages                        384
    rules for tag index pages (_tags/)           111
    pages that left with no rule                   0
    rules for an old URL that never existed        0
    rules over a page that still exists            0
    rules that land on no page                     0
    rules that land where the page did not go      0
  APPROVED  --  the claim checked here is about CONTENT

$ ./tools/checkpatch.sh -g e8a3d4ee1c...HEAD
✔️ All checks pass.

How you can check the "nothing changed" claim yourself.

I can push the script, the question is 'where'.
But how does it works? For every page outside the ten named above, it erases
what a move touches -- link target, path, tag line, toctree block, table border
-- from the whole old text and the whole new text, and requires the two to be
byte for byte identical. It exits non-zero and names the page if that is not
true, and it tests added pages too, so forgetting to declare one cannot make it
pass.