[FEATURE] Add a documentation site rendered with the Soul guides theme

[benjaminkott]

The project is documented in docs/ as reStructuredText and rendered with the TYPO3 Soul guides theme, in four sections: getting started, the Core checkout, a reference, and troubleshooting. The manual is written from the scripts rather than from the README, so it covers what each step actually reads and writes.

ddev docs renders it. The renderer is installed into .renderer/ on first use from the container's PHP, Composer and Node, so nothing is required on the host but DDEV, and the site is written to .site/. Both are dot-directories: generated, gitignored, never edited by hand.

Nothing is written into public/. That docroot belongs to TYPO3, and a generated directory inside it is something Composer and ddev tryout delete both have an opinion about. .ddev/apache/docs.conf maps /_docs/ onto .site/ instead, the same technique DDEV uses for /phpstatus. The leading underscore keeps the path clear of a page slug named "docs".

The post-start hook renders as its sixth step and prints the URL beside the backend one, so the manual is there the first time an instance is opened. The step is never fatal and TRYOUT_DOCS=0 skips it.

Two things the renderer does not do for us:

• --fail-on-error stops a render that cannot finish, but a :doc: reference that resolves to nothing is only a warning and still exits 0. Both ddev docs and the workflow read the output and fail on any app.WARNING line, so a dead link cannot publish itself quietly.
• The output directory is created 0700. Apache in the web container would get through as the mapped host user, but a mode that depends on which user the server happens to run as is a 403 waiting for another machine, so the build makes the site readable.

.github/workflows/publish.yml renders on every push and pull request and deploys to GitHub Pages from main. The renderer is built on the runner and thrown away with it; the repository holds documents and no manifest for them.