Update infrastructure document and move some content to contributing

[adrn]

I added a TODO item for @jonathansick to describe the contents of this repo, but otherwise described the current state of the infrastructure.

TODO for @adrn:

• Add information about the new Makefile in astropy-tutorials repo and how to build tutorials locally.
• Add information about pre-commit and how to set up locally if devs / contributors want.
• In dev workflow, always squash and merge
• In contributor workflow, if PR open, always pull changes before modifying further (because pre-commit.ci)

[eblur]

In the "add Information about pre-commit" check box, I would just like to see a set of command line instructions for the suggested workflow for modifying a repo branch.