Quality-of-live improvements for readme

What does the merge request do?

It improves the readme by obeying markdownlint rules, adding links and formatting where appropriate and improve general uniformity.

Taste is debatable, but I do believe this is an improvement in regards to following best practices.

Implementation notes

No implementation, this is about formal criteria of the readme.

Summary for release notes

Not noteworthy.

Checklist

Not applicable.