docs: add query docs for github (#467)

This PR adds detailed documentation for the query types supported by the
github data source

It introduces a summary table of all available types as well as details
on:
- query parameters
- sample queries
- response data

TODO:
- [x] review by @sympatheticmoose 

Notes for reviewers:
- Packages response is best effort based on `packages.go` - I was unable
to query this successfully
- Unsure on the `value` column for vulnerabilities, again took a guess.

Potential follow ups:
- Add screenshots for sample responses
- We could optionally extend this to cover specific permission
requirements on query
- Similarly I wonder if we should indicate whether a query respects
datetime picker in some form or not
- I got a little lazy with table formatting, this could be improved, but
never sure on best approach when column values are on the lengthy side

Author: sympatheticmoose