feat(page): every page renders as Markdown — .md format + copy-page button

[mhenrixon]

Closes #17. Part of #8 (Phase 3) — the keystone of the AI story: llms.txt, search, and MCP all consume this.

What & why

Every doc page is now also served as Markdown: GET /docs/x.md returns a faithful GFM twin of exactly what /docs/x renders, and a "Markdown" masthead action links to it. Authors write nothing extra — the twin is derived from the page's own render, so it can never drift from the page the way a hand-written to_text copy does (the app-4 status quo this kills).

How

We emit the HTML ourselves, so the tag vocabulary is bounded and a small hand-rolled Nokogiri visitor converts it exactly. Conversion happens post-render, so Phlex components, md islands, and raw tags in Prose all convert identically — no per-component parallel implementation to drift.

• DocsKit::MarkdownExport (+ Blocks/Inline/Table visitors): render the page → extract #docs-content → strip [data-md-skip] → walk to GFM. Handles h1–h4, p, a (relative hrefs absolutized via request base URL), strong/em/code, fenced code with language, nested ul/ol/li, GFM tables, > **Tip:** callout blockquotes, hr, img. Unknown elements recurse (text survives, wrapper dropped); script/style excluded.
• Render-time hints drive the tricky cases:
  • DocsUI::Code stamps data-md-lang (the resolved Rouge tag) → a ```lang fence
  • DocsUI::Callout stamps data-md-callout (the level) → > **Tip:**
  • DocsUI::Shell stamps id="docs-content" (the extraction anchor)
  • DocsUI::Page's "← Home" nav gets data-md-skip
• Wiring: DocsKit::Controller#render_page branches on request.format.md?/.text? → render plain: …to_md, content_type: "text/markdown". The engine registers the text/markdown MIME (:md). The install generator route allows the (.:format) segment.
• Copy affordance: DocsUI::MarkdownAction — an <a href="….md"> (JS-off: opens raw markdown) that docs-nav enhances into copy-to-clipboard (a new copyMarkdown action + markdownLink target on the one controller — the single-controller rule holds). Config c.page_markdown_action (default true) opts it out.
• Dependency: nokogiri added to the gemspec (already present in every Rails host; no Gemfile.lock change).

What a consuming site gets

Delta	Where
GET /docs/x.md → the page as GFM	render_page + the (.:format) route
A "Markdown" masthead action (copy-to-clipboard)	DocsUI::Page + DocsUI::MarkdownAction
c.page_markdown_action knob (default true)	DocsKit::Configuration

Existing sites re-run docs_kit:install (or add (.:format) to their get "docs/:doc" route) to enable the .md URLs. Sites that don't re-run simply have no .md route match — HTML rendering is untouched (backwards compatible).

Test plan

• spec/docs_kit/markdown_export_spec.rb — semantic asserts over the full vocabulary: heading levels, fenced language, absolutized links, nested-list indentation, GFM table row, > **Tip:**, chrome / [data-md-skip] / script excluded, plus a full-page acceptance fixture.
• spec/docs_kit/controller_spec.rb — .md/.text render the text/markdown twin; an HTML request still renders layout: false.
• spec/docs_ui/code_spec.rb / callout_spec.rb — the data-md-lang / data-md-callout hints.
• spec/docs_ui/markdown_action_spec.rb — the .md href (query-safe, idempotent) + the docs-nav enhancement wiring.
• spec/docs_kit/configuration_spec.rb — page_markdown_action default + override.
• spec/generators/install_generator_spec.rb — route allows (.:format), no format: "html" pin.

Verification

• bundle exec rspec — 297 examples, 0 failures (95.23% line coverage)
• bundle exec rubocop — no offenses
• Dogfood: a realistic page (header, section, md island, code, callout, prop table, nested + ordered lists, relative link) → clean GFM
• docs-nav JS: node --check passes; copyMarkdown degrades to the plain link when clipboard/fetch is unavailable
• Backwards compatible: no .md route match → HTML rendering untouched