docs: make the README a landing page and link out for reference - #587

This serves the Commit Check badge from the site, so the snippet a user
copies becomes one line:

```markdown
[![commit-check](https://commit-check.com/badge.svg)](https://commit-check.com)
```

Today the snippet is a 519-character shields.io URL with the mark
base64-encoded into it, and every page view makes a request to a third
party. conventionalbranch.org/badge.svg already works this way.

## What's in it

- **`docs/badge.svg`.** This is a hand-written copy of the shields.io
render of the brand badge from
[`branding/README.md`](https://github.com/commit-check/.github/blob/main/branding/README.md).
It has the same 157×20 size, the same colors and flat style, and the
same `textLength`, which keeps the text identical on machines without
Verdana. The mark is drawn as vectors instead of an embedded image. A
comment in the file explains all of this, including the contrast
trade-off: white on Signal Blue is about 3:1, which is the color the
badge has always had. MkDocs copies the file to the site root.
- **Getting started.** A short *Show that you use it* section now shows
the badge, with Markdown and reStructuredText snippets in tabs. Until
now only the READMEs documented the badge.

## Checks

- I rendered both files with `rsvg-convert` at 6× and compared them with
ImageMagick. At 8% fuzz, 0 pixels differ, and the RMSE is 1.5e-5.
- `mkdocs build --strict` passes (with `SOCIAL_CARDS=false`).
`site/badge.svg` is in the output, and the page links it as
`../badge.svg`.
- `python -m pytest tests/ -q` passes with the released commit-check
installed (10 tests). The pins and the changelog were already at 2.18.1,
so nothing needed bringing into step.

## After this deploys

Once `https://commit-check.com/badge.svg` is live, the long snippet
elsewhere can switch to the short one:
- the *Show that you use it* sections of the commit-check and
commit-check-action READMEs (commit-check/commit-check#587,
commit-check/commit-check-action#293);
- the *README badge* section of `branding/README.md` in `.github`.

The badge rows in those READMEs can stay on shields.io. Only the snippet
users copy needs to change.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Added a “Show that you use it” section with a commit-check badge and
copyable Markdown and reStructuredText snippets.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

This adds a Python API page. `commit_check.api` had no page on the site.
The core README used to document it, until the README pass
(commit-check/commit-check#587) cut that section down to one example on
the understanding that the site would take it over. This is that page.

## `guides/python-api.md`

- **Functions.** It covers all six, in one table: `validate_message`,
`validate_branch`, `validate_author`, `validate_tag`, `validate_push`
and `validate_all`. The old README never listed `validate_tag` or
`validate_push`. For each function the table says what it checks and
what it reads from git when a value is left out. `validate_all` reads
nothing from git.
- **Result.** The shape is the same as `--format json`. The page
explains what `pass`, `fail`, `skip` and `warn` mean, and when `fix` is
filled in.
- **Configuration.** The API does **not** read `cchk.toml`. `config` is
a dict merged over the built-in defaults. To use a repository's own
policy, load the file with `tomllib` and pass it in. On Python 3.10,
commit-check already depends on `tomli`. `inherit_from` is not followed
this way, and the page says so.
- **Fixing until it passes.** A short example applies `fix` and
validates again, which is the same loop the MCP server teaches.

## Links to the page

- The nav, under *Guides*, after *MCP server*.
- *Where to run it*: a row in the table and a short section.
- The end of *Reading the JSON* in *Command-line recipes*.

## Checks

- I ran every Python sample on the page against the released
commit-check **2.18.1** and pasted the output as printed. A script
re-ran each block and diffed it against the page, and all of them match.
I also checked that `validate_tag()` returns `skip` on a commit that has
no tag, as the table says.
- `mkdocs build --strict` passes (with `SOCIAL_CARDS=false`).
- `python -m pytest tests/ -q` passes with the released package
installed (10 tests). The pins and the changelog were already at 2.18.1.

## Follow-up

Once this deploys, the core README's *AI-native usage* section can link
here. That section currently names only three of the other five
functions.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Added a Python API guide covering how to run commit checks in-process,
configure checks, interpret results, and apply suggested fixes.
* Added the Python API to the integrations comparison and documentation
navigation.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->