Migrate documentation from Sphinx to Markdown and Jekyll

[driv3r]

TL;DR

Rewrite the reStructuredText docs as plain Markdown under docs/, so they read directly on GitHub, and build the site with Jekyll and the Just the Docs theme instead of Sphinx. This should make it easier to extend documentation, whenever view on github or github-pages

Important

There are no changes to the content itself, just the format and the engine. Reviewer should keep this in mind. Making docs up to date will be in a follow up.

Summary

• Keep existing page URLs (introduction.html etc.) and every public section anchor, including explicit labels such as #prodtesting.
• Move _static assets unchanged; restore the two-level contents list on the home page.
• Titles, navigation order and theme settings live in docs/_config.yml, so the Markdown files need no front matter.
• Tokyo Night colour scheme (Night and Day) with matching Rouge syntax colours. Follows the system light/dark setting, with a header toggle (System / Light / Dark) saved in localStorage; selection happens in a blocking head script so pages never flash the wrong theme.
• Add a :docs Gemfile group (jekyll, just-the-docs, html-proofer); add x86_64-linux and ruby lockfile platforms for CI.
• Documentation workflow: build and link/anchor-check on every PR and push, upload the site as the docs-html artifact, and deploy only that artifact to gh-pages/main from pushes to main. Other published versions and the root version selector are left untouched.
• Ruby test job skips the docs gems (BUNDLE_WITHOUT development:docs).
• dev docs / dev docs-build commands and README instructions.
• Remove the Sphinx build script, Makefile and conf.py.

New layout

Light

Dark

Live Search

Tophat

1. Checkout the branch
2. dev up
3. dev docs
4. visit http://127.0.0.1:4000/ghostferry/main