Mass conversion of documentation from rst to Markdown
# What does this PR do?

This PR treats a lot of docstrings and converts them from rst to Markdown. All of the remaining one.

It also includes a new repo consistency check that will detect someone does not add back some rst docstrings behind my back. 😈 
As seen with @LysandreJik, merging and inspecting the result in the doc. Failure is unrelated.