site/api-md: Prerender markdown routes at build time; enable Fluid compute

[marcleblanc2]

Vercel audit items 5a and 5b (see #1905, dev/TODO.md item 5). Stacked on #1901 because both edit vercel.json; merge #1901 first, then retarget this to main.

What changes

/api/md/[...slug] (serves <page>.md, which the middleware rewrites to this route):

• generateStaticParams() returns every post's slug, so Next writes each markdown response to disk during next build (.next/server/app/api/md/**/*.body). Vercel serves those from the CDN: no function invocation, no cold start.
• dynamicParams = false makes unknown slugs 404 at the edge without invoking a function. Behaviour change: an unknown .md URL now gets the site's HTML 404 page (like any other unknown URL) instead of the plain-text Not Found the function used to return.
• The root index.mdx is skipped: its flattened path is empty and a catch-all needs at least one segment. /api/md was never a valid URL for this route; the first build attempt failed on exactly that.

vercel.json: "fluid": true turns on Fluid compute for the remaining functions (/api/og/[...path], the [...slug] page renderer for non-prerendered paths). Fluid lets one instance handle many concurrent requests instead of one cold start per request, and bills active CPU time rather than wall-clock. Memory/CPU size still lives in the dashboard (Settings → Functions), not in vercel.json.

Verification (local)

• pnpm build: /api/md/[...slug] shows as ● (SSG) with ~500 paths; build time unchanged (about 40 s locally).
• pnpm start, then:
  • /api/md/admin/config/site-config → 200 text/markdown, 47 902 B
  • /api/md/cli/references/auth → 200 text/markdown, body starts with # src auth
  • /api/md/does/not/exist → 404

Verification (after the preview deploys)

Compare against production for the same URL:

for u in https://sourcegraph.com/docs/admin/config/site-config.md <preview>/docs/admin/config/site-config.md; do
  curl -so /dev/null -w '%{http_code} %{time_total}s\n' -D - "$u" | grep -iE 'x-vercel-cache|^[0-9]{3} '
done

Expect the preview to answer with x-vercel-cache: HIT (or PRERENDER) and a faster time_total on cold paths. For /api/og/..., expect similar or better latency under Fluid; nothing should be slower.

[vercel]

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Project	Deployment	Actions	Updated
sourcegraph-docs	Ready	Preview	Sep 11, 2026 5:59am UTC