Docs: added "Cheat Sheet"

[scottrippey]

Adds a "Cheat Sheet". This document was copied from Sanity's own Cheat Sheet docs and adapted for GroqD.

Testing

Review this doc here:

https://groqd-git-cheat-sheet-formidable-labs.vercel.app/docs/API/groqd-cheat-sheet

TODO:

• [x] Organize better via frontmatter
• [ ] Consider eliminating irrelevant examples (eg. all the raw)