Improve build times for entire corpus

[Blargian]

This changes the documentation build architecture so English and each requested translation compile in isolated, sequential Astro processes before being merged into one Vercel artifact. This bounds peak memory, retains a single deployment, keeps ordinary previews scoped and fast, and still allows production to include every locale.

The build now:

• compiles only the active translated collection in each locale process;
• generates missing-translation fallbacks from the English process;
• emits one canonical English .md agent representation per page and no duplicate .mdx or localized agent surfaces;
• parses and indexes sidebar navigation once per locale;
• uses namespaced locale assets and stable incremental-build cache keys; and
• reports preview deployment state and selected components through a single sticky pull-request comment without allowing comment failures to interrupt Vercel polling.

The full production-shaped build completed in 18 minutes 44 seconds. Type checking, combined English and Spanish output generation, incremental cache restoration, workflow YAML validation, and shell syntax validation also passed.

Changelog category (leave one):

• Build/Testing/Packaging Improvement

Changelog entry (a user-readable short description of the changes that goes into CHANGELOG.md):

Reduce documentation build time and peak memory by compiling localized content in isolated sequential Astro processes and merging the results into one deployment.

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Stale Bugbot comment from a previous run.

[github-actions]

Nimbus documentation preview (trusted-branch, English only): https://clickhouse-docs-1qqugsof0-clickhouse.vercel.app

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Stale Bugbot comment from a previous run.

[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 2 potential issues.

There are 3 total unresolved issues (including 1 from previous review).

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^{Reviewed by Cursor Bugbot for commit 10f3e7b. Configure here.}