[Docs] Add a Community extension page

[dhmlau]

Add a community extension page to list extensions built by the community. It will be similar to the community connector page: https://loopback.io/doc/en/lb4/Community-connectors.html.

Acceptance criteria

• add a community extension page with a few community extensions we know about.
  • the idea is to have the community user to submit PRs to add their extensions on this page
• add this to sidebar of the docs
• Add link to npm keyword "loopback4-extension" page.
• Add links to pages about creating extensions.

cc @raymondfeng @bajtos, per our discussion today.

[dhmlau]

We might be able to use some of the extensions listed on npmjs: https://www.npmjs.com/search?q=loopback4-extension.

@samarpanB @ckoliber, I believe you had a few LoopBack 4 extensions. Please feel free to add here to help us building the initial list!

[samarpanB]

Hi @dhmlau ,

Here are the ones we created

1. https://www.npmjs.com/package/loopback4-authentication
2. https://www.npmjs.com/package/loopback4-soft-delete
3. https://www.npmjs.com/package/loopback4-authorization
4. https://www.npmjs.com/package/loopback4-helmet
5. https://www.npmjs.com/package/loopback4-ratelimiter
6. https://www.npmjs.com/package/loopback4-notifications
7. https://www.npmjs.com/package/loopback4-s3

[ckoliber]

Hi @dhmlau ,

Here are my extensions:

1. loopback-component-history: Soft create, edit, delete models, saving history of changes
2. loopback-component-filter: Filter models in repository layer, using an async function
3. loopback-component-crud: Generic CRUD controller mixin, supports authentication, nested authorization(cascade)
4. loopback-component-cascade: Create, update, delete nested models in repository layer
5. loopback-component-authorization: HRBAC authorization implementation