Document migration steps for changed rules for given prioritisation under 3.6

[WojciechMazur]

Scala 3.5 introduces gradual changes to rules prioritization #19300 , these now (3.5) warn about selected implicit, and would start yielding errors in the future (3.6).
These changes lead to multiple warnings/errors in real-world projects like lichess-org/lila. We should identify the patterns in which warnings occur and provide a guide/migration steps on how to deal with them. Especially how to deal with changed resolved targets.

[soronpo]

The main issue with this change is that it's a problem that manifests down to the application level. We cannot simply just silence the warnings at the library level. Meaning that any library that experiences this, now needs its own migration guide for its users.