Migration guide: mount LB3 in LB4

[bajtos]

This is a follow-up for the spike #2318, see the discussion and PoC code for more context.

Create a migration guide for LB3 users showing them how to mount their existing LB3 project into a new LB4 app.

Related issues: #1849 #2389 #2391 #2392

Acceptance criteria

• Start by reorganizing the docs for LoopBack 3.x users: -> docs: add mounting lb3 app on lb4 migration guide #3052

  • Keep the current page as a top-level overview.
  • Move most of the content into a new page with a name like "Understanding the differences".

• Create a new page with a name like "Migrating from v3". Describe how to use Lb3AppBooterComponent. Explain the differences (breaking changes) introduced by the migration. ATM, we are aware of a single difference: the spec format. LB3 produces Swagger (OpenApi v2), LB4 produces OpenApi v3. -> docs: add mounting lb3 app on lb4 migration guide #3052

• A blog post to announce this new migration option -> https://github.com/strongloop/strongloop.com/pull/203

[nabdelgadir]

Closing as done.