ci: add docs build verification workflow

[sentry-junior]

The dex-docs Astro site (packages/docs) is not verified in CI. A broken docs build can reach production undetected.

Add a separate docs.yml workflow that runs in parallel with the main CI job:

• Same setup: pnpm, Node 24, frozen lockfile
• Run pnpm build first (docs depend on @sentry/warden via workspace:*)
• Run pnpm --filter dex-docs build to verify the Astro site compiles
• Trigger on push to main/release/* and PRs to main
• Only needs contents: read permission

Note: the GitHub App token lacks the workflows scope, so this file must be pushed manually or via a PAT.

Action taken on behalf of David Cramer.