dbt debug changes

[VersusFacit]

Contributions

• I have read the contribution docs, and understand what's expected of me.

Link to the page on docs.getdbt.com requiring updates

This page is missing a ton of things about debug!

What part(s) of the page would you like to see updated?

1. The "complete purpose" of debug -- that it checks user configuration for local runs of dbt on a number of axes.
2. Flags like --connection which bypass execution of some steps
3. What kind of system info (e.g. os, python, etc.) and dbt info (e.g. adapter version) is included
4. Other flags uinque to dbt debug (some of which may be added later)

Additional information

This PR and it's issue details most of the recent work and why we chose it.