How to migrate model relations

[bajtos]

This is a follow-up for #3718 and #3922.

Write content for docs/site/migration/models/relations.md, explain how to migrate a relation defined in LB3 model JSON file to a LB4 app.

• Relation definition to LB4 model class
• Persistence (via LB4 Repositories), including inclusion resolver
• REST API (via LB4 Controllers)

Acceptance criteria

• Migration guide describing manual steps
• Explain that we are working on simplifying relation setup via declarative approach, link to the following GH issue: From relation definition to REST API with auto-generated repository/controller classes [EPIC] From relation definition to REST API with auto-generated repository/controller classes #2483
• A table showing which LB3 relation types are supported by LB4 (e.g. HasMany, BelongsTo) and which relations are not supported yet (e.g. ReferencesMany). For each relation that is not supported, provide a link to the tracking GH issue. See also [EPIC] More Relations (post-GA) #1450