Apply linter to the repo and update rules

[egrace479]

Applied the linter to the repo and adjusted the rules where needed:

• Set to ignore mkdocs.yaml and our templates -- these have particular formatting we don't want edited
• Actually allowed "!" in headers.
• Allowed code blocks without language specification.
• Allowed fenced code blocks, as this commonly errors when indented (see discussion).
• Manually disabled rule for table on part of the command line cheat sheet because it was being trigger through improper recognition by markdownlint (and VS Code), though it renders properly on the site.
• Manually disabled rule for in-line HTML on the command line cheat sheet page since it is needed for better table formatting and brackets aren't recognized as being in a code block by the linter.

For the record, the linter has consistency within a file, not as much across files (e.g., - vs * as a bullet point). We should now be at the point where an action could be added without causing a major headache.

[hlapp]

Looks good!

[thompsonmj]

👍