Create Granite documentation

[faustinoaq]

Hi @drujensen I think Granite's README is growing very fast, maybe we should maintain a separate directory (or repo) for docs like Crecto and Jennifer do, WDYT?

BTW, I already created a GitBook page for Granite here: https://amberframework.gitbook.io/granite (linked to this repo and using Granite's README)

Update: new link 🎉 https://docs.amberframework.org/granite/

[Blacksmoke16]

At the very least a table of contents would help.