[ForgeGradle] The ForgeGradle 6 Update

[ChampionAsh5357]

Adds the docs for ForgeGradle 6. Currently waiting on some comments from Shrimp about additions and removals that should be included.

• [x] Add details on new properties in Minecraft extension
• [x] Add details on libraries in reobf config task

[ChampionAsh5357]

I'm just going to mark this as ready since FG6 is out and there haven't been any more changes.