Expose per-page Markdown on the docs site

[dahlia]

VitePress + vitepress-plugin-llms already generates per-page Markdown during the docs build, but there's no UI to reach it. Readers currently have to know about llms.txt or guess the generated file path.

Add a per-page action that lets readers view, copy, or download the generated Markdown for the current page.

What we already know:

• Fedify docs already use a custom VitePress theme entry at docs/.vitepress/theme/index.ts, so this can be added without restructuring the docs site.
• The docs config already enables vitepress-plugin-llms in docs/.vitepress/config.mts, so the Markdown-oriented output is already part of the build pipeline.
• VitePress supports extending the default layout with custom components and layout slots, which should be enough for a small per-page control.
• vitepress-plugin-llms appears to provide built-in per-page Markdown buttons/components. If they fit the current theme, prefer using them over a bespoke implementation.
• If the built-in component is not a good fit, a small wrapper that links to the generated Markdown for the current page should still be feasible.

Constraints:

• The control should fit the existing docs theme instead of introducing a new visual pattern.
• It should not disturb the page title area, the outline, or the mobile layout.
• The docs site should still build successfully with mise run docs:build.

Done when:

• Each documentation page exposes an obvious action for its Markdown form.
• That action resolves to the correct generated Markdown for the current page.
• The control works on both desktop and mobile layouts without overlapping existing UI.