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.

Start by reading update-blog.sh, update-readmes.sh, and update-lb4-docs.js, then inspect how npm run build invokes them and how CI handles the generated assets. Define the submodule locations and the built paths that should be ignored. Done means the vendored sources have clear Git provenance, builds no longer overwrite tracked generated copies, and the existing site build still works.