Convert docstrings of modeling files
# What does this PR do?

This is going to be a huge PR (sorry) but all modeling files have docstrings that are connected to the model outputs and the code samples in file_utils, so the conversion from rst to Markdown has to happen for all of those at the same time.

The doc-styler is commented out for now, as it can't deal with example written in Markdown. I will work on re-enabling it as soon as we have finished converting all the documentation.

Separately, this PR deals with an issue of the Return block in some dosctrings being indented at 4 spaces instead of the indentation inside the docstring, which resulted in the Example block as being part of the return block.
Saw with @sgugger and the newline before `"""` isn't important and isn't liked by black.

As seen with Sylvain and Patrick, merging this PR now to prevent any conflicts - let's take a look at `master` once deployed to ensure everything works correctly.<|||||>From what I'm seeing, everything looks correct, including code samples! Thanks for your work, @sgugger!