use standard markdown lint rules

[themightychris]

Per this discussion #48 (comment), adjust the markdown linting configuration to follow defaults entirely.

The one rule I recommend overriding is setting list indentation to 4 spaces, as this is often important for mkdocs to correctly render content embedded within list items like code fences and admonitions.

See #52, markdown-linter-configuration.yml should be brought in line with this single override