add markdownlint pre-commit checks and fixup warnings

This MR builds on !262 (closed) by fixing up markdownlint issues.

Add pre-commit checks which use markdownlint to check markdown files in the repo. This exposed a great number of issues which have mostly been automatically fixed. Line length issues have been addressed by either re-wrapping the lines or, in the case of genuine long longs, by explicit disabling of the line-length check.

Also closes #222 (closed).