Style guide for written text

There should be a style guide for the documentation, to keep things more consistent, even if it can't be machine checked.

Good points:
* Avoid jargon when possible, link to definitions if possible
* We should standardi(s/z)e on a particular English variant (or at least ask that any given document is self-consistent)
* Capitalize acronyms
* Put command-line tools, programming-language identifiers, and package names in `code font`
* Annotate fenced code blocks with their language where possible
* Use `$` to indicate shell prompts
* etc

Ideally, we can do some of this through CI - I'll look around at Markdown-aware style checkers.