Document how to mount Git repositories at runtime via git-pull config

[choldgraf]

This adds documentation for hub administrators to learn about mounting Git repositories to user environments at startup. This feature is already available and implemented for communities (like NASA VEDA and EarthScope), but was previously only documented in our engineering docs.

Two extras:

• Along the way I also created minimal viable documentation guidelines as a standalone page (contribute/mvp.md) and I'll link to that from our compass so it can be the source of truth.
• I also refactored the noxfile to use separate docs and docs-live sessions instead of docs -- live

Closes:

• Closes Document how to mount folders and repositories to your user image at runtime via nbgitpuller #278