DOCS: Add information about installing on macOS arm64. General clean up.

[baagaard-usgs]

• Consolidate architecture-dependent notes in the installation instructions.
• Convert initial_setup.rst to Markdown.
• Fix typos (misspelled words).
• Reformat with each sentence on its own line.

[cbworden]

Again, I am concerned with the idea of putting each sentence on its own line. I think this needs to be discussed and reviewed before it becomes policy.

[baagaard-usgs]

I agree that one sentence per line is slightly more cumbersome to write, but it makes the git commit history much, much cleaner and easier to follow. Most editors can wrap long lines to whatever length you want. I don't plan on reformatting any additional files.

[cbworden]

There does seem to be some support for the one-sentence-per-line approach. There is also support for the "semantic linebreak" approach, wherein you break lines at thoughts, ideas, clauses, etc. I'm in favor of just breaking lines at the natural line limits, because it works cleanly for the most editors and the most environments without horizontal scrolling. But maybe we should put an intern to research it and see what the CW says we should do in 2022.

[emthompson-usgs]

I'll just say that I don't have a strong opinion on the line style issue. I am more accustomed to what we've been doing, which is what Bruce is advocating for. I don't tend to interact with the git commit history very much and so I don't think I currently appreciate this benefit so it is difficult for me to comment on.