feat: add navigation header to per-page .md files

[marekh19]

Resolves #2557

Prepends a small frontmatter block to every per-page .md served by docs.apify.com so AI agents reading a single page can follow parents / children / previous / next links without having to fetch llms.txt first. Shape modeled on Vercel's docs (vercel.com/docs/builds/build-features.md): flat YAML-style frontmatter, no nested trees, links are inline [Title](url.md) so the agent can pick by title without a probing fetch.

Implemented as scripts/addNavHeaders.mjs in the postbuild chain. Runs after @signalwire/docusaurus-plugin-llms-txt writes both the per-page .md files and llms-full.txt, so editing only the per-page files leaves llms-full.txt untouched (the issue requires the header to stay out of it). Reads the real left-menu trees from .docusaurus/docusaurus-plugin-content-docs/*/p/*.json, so labels, ordering, and prev/next exactly match the live sidebar.

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A few design decisions worth recording:

Header shape — modeled on Vercel's docs: flat YAML-ish frontmatter, links as inline [Title](url.md) so an agent can pick by title without a probing fetch.

Children stay flat (direct only). A full recursive sub-tree explodes on exactly the pages an agent lands on first — /api/v2 would carry 269 nested links, /platform/integrations 66, /platform/actors 57. That pollutes the header and bloats neighbouring content for no real gain: the agent can walk down one hop at a time via each page's own children.

Parents go all the way up. The opposite trade-off — a breadcrumb is a single chain bounded by site depth (~5 max), so there's no blow-up risk. Every page now chains llms.txt root → section landing → category ancestors, giving a reliable "zoom back out" path. This also fixes 83 top-level pages that previously had no parents at all.

Kept out of llms-full.txt for free by running in postbuild, after the llms-txt plugin has already written llms-full.txt from the same per-page content.

previous/next links mirror the site's prev/next buttons.

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Example 👀

https://pr-2596.preview.docs.apify.com/platform/actors/running.md

[apify-service-account]

✅ Preview for this PR (commit e1d0103e) is ready at https://pr-2596.preview.docs.apify.com (see action run).