Minor updates to documentation files

[robertbartel]

A few fixes (e.g., typos), refinements, and updates to the ./doc/GIT_USAGE.md and ./INSTALL.md documentation files.

[christophertubbs]

Can you add a blurb about setting up a PR on here? Maybe something mentioning preferred uses of draft PRs for easier collaboration?

Something saying that github tests should pass before approving PRs (except for extenuating circumstances, such as upstream dependency issues at the time of writing) and something saying that "Rebase and Merge" button should be clicked instead of just the standard "Merge" button and where to find it.

If you say no I'll go ahead and approve anyways.

[robertbartel]

I've made another issue for it in our board. We may not have time to get to that before things next week, but I'll convert unfinished any such things to full issues and keep them on the project board.