Move vendored assets to Git Submodules and `.gitignore` built assets

The website has scripts which pulls data from GitHub and NPM:

• update-blog.sh
  Clones loopbackio/loopback-blog into /blog
• update-readmes.sh
  Downloads READMEs from GitHub "raw" or NPM Registry into /doc/en/[lb3|lb4|community]/readmes
• update-lb4-docs.js
  Copies pre-generated markdown API Docs from @loopback/docs into /doc/en/lb4

They pose some issues:

• Accidental manual editing: Contributors may accidentally contribute changes to these assets, only to be overridden by the CI pipeline.
• Rate-limitng: Contributors running npm run build -> update-readmes.sh may inadvertently update the local copies with GitHub "rate limit reached" error pages.
• Unclear provenance: As the .git directory is not preserved, the provenance of where and when exactly the assets were vendored is no clear.
• Updates are reactive: The assets are only re-vendored when a PR is merged.

Git Submodules solve this problem. Updates would be handled by Renovate instead, which would open PRs.

Commencez par lire update-blog.sh, update-readmes.sh et update-lb4-docs.js, puis examinez comment npm run build les invoque et comment CI gère les assets générés. Définissez les emplacements des sous-modules et les chemins de build qui doivent être ignorés. Le travail est considéré comme terminé lorsque les sources vendorisées ont une provenance Git claire, que les builds ne remplacent plus les copies générées suivies par le contrôle de version et que le build existant du site fonctionne toujours.