pre-commit: add markdownlint and standardize Markdown files

[jbampton]

Popular tool to standardize a projects Markdown

https://github.com/DavidAnson/markdownlint

Using the CLI with pre-commit:

https://github.com/igorshubovych/markdownlint-cli?tab=readme-ov-file#use-with-pre-commit

So we now have another basic check / hook / test running with pre-commit.

Nice to have standardized and formatted Markdown

Just ignored a small number of rules for now:

https://github.com/DavidAnson/markdownlint?tab=readme-ov-file#rules--aliases