ci(docs): deploy docs to Netlify, bump pnpm/action-setup to v6

[luishcastroc]

What

• Rewrites the docs deploy workflow to publish to Netlify via netlify-cli instead of GitHub Pages, which was failing with a 404 (Ensure GitHub Pages has been enabled) since Pages is not enabled on this repo. Drops the Pages-specific pieces: pages/id-token permissions, the deploy-pages job, the 404.html SPA fallback copy, and the DOCS_BASE=/angular-gsap/ base path (Netlify serves from the domain root).
• Adds apps/docs/public/_redirects (/* /index.html 200) as the Netlify SPA fallback; prerendered pages still win because real files are served before redirects.
• Bumps pnpm/action-setup v4 → v6 in both workflows: v5 moved the action to Node 24 (clears the Node 20 deprecation warning), v6 added pnpm 11 support, which this repo uses.
• Docs fact-check fixes: home page tree-shaking card said 9 other directives (there are now 12 total), README size claim now says what was actually measured (~6 kB min+gzip), and the reference page gains the missing effects row for provideGsap.

Setup required before the deploy works

The workflow needs two repo secrets: NETLIFY_AUTH_TOKEN and NETLIFY_SITE_ID. Until they exist the deploy step will fail on auth.

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