📝 docs: fix trailing-slash links left by the Sphinx move - #1986

Closes #1980.

The Release Notes link in pyproject.toml is the one @kevinbowen777 reported, and it turned out not to be alone — the same MkDocs directory-URL shape survives in six more places. MkDocs served every page as a directory, so a link to one ended in a slash; #1957 moved the site to Sphinx, which emits <page>.html, and Read the Docs serves the built tree as is.

All seven, found by scanning rather than by eye:

pyproject.toml                    /latest/changelog/
.github/CONTRIBUTING.md           /latest/contributing/
.github/PULL_REQUEST_TEMPLATE.md  /latest/contributing/
docs/README.md                    /stable/explanation/comparisons/
docs/README.md                    /stable/how-to/install-pipx/          (x2)
docs/README.md                    /stable/contributing/

pyproject.toml and docs/README.md both feed the PyPI project page — the sidebar link and the long description respectively — so five of the seven are visible to anyone who lands there.

Checked against the live site rather than assumed: /latest/changelog.html and /stable/how-to/install-pipx.html answer 200, /latest/changelog/ answers 404.

The test

tests/test_docs_urls.py scans tracked documentation and pyproject.toml for the old shape so it cannot come back. Two notes on how it is built, since both were mistakes I made first:

• It reads the tree with pathlib instead of shelling out to git ls-files. A subprocess call needs git on PATH, which is not guaranteed for whoever runs the suite.
• Its terminator set includes quotes. My first version only matched a bare URL or a markdown link, so it silently passed over the one link the issue is actually about — the URL in pyproject.toml is inside a TOML string and ends at a ".

It needs neither network nor the package cache, so it runs anywhere the suite runs.

• I have added a news fragment under changelog.d/
• I have added a test