Need to be sure to update / highlight the game data dir in doc everywhere

[Cervator]

Especially as users start migrating to the multiplayer branch

• Help users find and delete (or provide a button / command of some sort?) any outdated directories
• Make sure Gradle knows how to deal (like the utility task for clearing out game data)
• Update GitHub wiki
• Update README and anywhere else we can think of

Latest dir on Windows is something like C:\Users\Cervator\Saved Games

[Cervator]

Linking to MovingBlocks#520

[Cervator]

Also that for dev you need the latest Git version or the "**" pattern won't work - not super relevant here but putting the note in multiple places so I won't forget 🍴