Migrate doc files to Markdown.
# What does this PR do?

The new UI in GitHub makes MDX pretty hard to read for diffs, so this PR migrates the doc files from mdx to md. This shouldn't break anything in the doc-builder.
I'd add a :warning: emoji as a prefix for the disclaimer, but other than that it looks good to me!<|||||>_The documentation is not available anymore as the PR was closed or merged._