Missing migration guide for v1.5

[3flex]

There are migration guides for v1.1, 1.2 and 1.4, but they're missing for 1.3 & 1.5 (though v1.3 is less relevant now since it was release 2 years ago, so it's probably not worth writing anything up now).

v1.5 has breaking changes (no longer supports Jellyfin 10.8, changes to API, maybe more) which would ideally be documented in a dedicated migration guide.

[nielsvanvelzen]

There are 2 main changes in 1.5:

• The new WebSocket API
• The API is updated to 10.9

I don't document API changes in migration guides, for the migration of the WebSocket implementation you can just look at the updated documentation for it, it's not that different.

I didn't had the time to write a proper migration guide during the 1.5 release, I do accept pull requests for it though!