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

まだ誰も着手していません。

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

- [`update-blog.sh`](https://github.com/loopbackio/loopback.io/blob/dc08f345c51fcf85cef04ad55c2a56e5812f2451/update-blog.sh)
Clones `loopbackio/loopback-blog` into `/blog`
- [`update-readmes.sh`](https://github.com/loopbackio/loopback.io/blob/dc08f345c51fcf85cef04ad55c2a56e5812f2451/update-readmes.sh)
Downloads READMEs from GitHub "raw" or NPM Registry into `/doc/en/[lb3|lb4|community]/readmes`
- [`update-lb4-docs.js`](https://github.com/loopbackio/loopback.io/blob/dc08f345c51fcf85cef04ad55c2a56e5812f2451/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](https://github.com/renovatebot/renovate) instead, which would open PRs.

まず update-blog.sh、update-readmes.sh、update-lb4-docs.js を読み、次に npm run build がそれらをどのように呼び出しているか、また CI が生成されたアセットをどのように扱っているかを調べます。サブモジュールの場所と、無視すべきビルドパスを定義します。ベンダー化されたソースの Git 上の出自が明確になり、ビルドによって追跡対象の生成コピーが上書きされなくなり、既存のサイトビルドが引き続き動作すれば完了です。

索引モデルが issue の本文から書いたものです。