[7.0] Write proper documentation

[Jonathing]

ForgeGradle 7.0 and later need proper documentation to avoid confusion (as demonstrated by #1026 and similar issues) and to better showcase the functionality that it provides.

Important

I've made the executive decision to use Docusaurus for all of our Gradle plugins. If it goes well enough, I may consider migrating our current Forge docs from Read the Docs to Docusaurus, though no guarantees on that. I will push for projects that do not have existing documentation to use Docusaurus. More on this at another time.

[Jonathing]

cc @Fealtous

[LexManos]

How are we to host these files? Docusaurus shows examples of hosting via github pages.
I could setup a DNS record for a github page. However I am more inclined to host the files ourself as they are just static generated files.
Doing so would allow us to provide good 'all projects' index pages. As well as have different versions of the docs for different branches.