How to leverage "booting an app using component" in extensions

[bajtos]

#5304 added a helper allowing applications to load artifacts from other LB4 applications, see
https://loopback.io/doc/en/lb4/Booting-an-Application.html#boot-an-application-using-component

This concept can be useful to extension authors too, let's update our documentation to explain that in more details.

Related:

• docs: create REST API endpoints in components + LB3 migration docs: create REST API endpoints in components + LB3 migration #5385

Acceptance criteria

• Add content explaining when and how to leverage the concept of app composition in new LB4 extensions

  For example, we can add a new page to Extending LoopBack or expand one of the existing pages

• Add similar content for people migrating a LB3 extension to LB4, see Migrating components and extension. Don't repeat yourself, the migration guide should focus on aspects unique to extensions migrating from LB3 and refer to content created in the step above wherever possible & appropriate.

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[stale]

This issue has been closed due to continued inactivity. Thank you for your understanding. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository.