How to migrate datasources from LB3 to LB4

[bajtos]

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

Write content for docs/site/migration/datasources.md, explain how to migrate datasources from a LB3 project to LB4.

For example:

1. Run lb4 datasource, enter the same name as in LB3, pick the connector used by LB3 datasource
2. Enter default values for all prompts
3. Copy datasource configuration from lb3app/datasource.json to src/datasources/{name}.datasource.json

Acceptance criteria

• Migration guide describing manual steps
• A section explaining that LB4 is compatible with existing LB3 connectors
• Create a new issue to implement automated import (e.g. lb4 import-lb3-datasources)