Markdown support

I'm currently moving our tutorial notebooks/documentation from RST with jupyter-sphinx to Markdown with myst-nb. We'd eventually like to also move docstrings to be written in myst-markdown. To that end I'm now looking at what I actually need from sphinx-autoapi to make that possible. I came up with this:

1. Cast the existing templates into Markdown. Includes using a (potential) sphinx.util.docstrings.prepare_docstring analog from myst. This is to allow two sets of default templates to exist.
2. Make the hardcoded '*.rst' instances in template loading/rendering (mappers/base.py) use some config value instead. The actual file output suffix is already (not really explicitly) configurable.

I'd be happy to contribute this. If I was to do this just for our project, I'd still have to do step 1., but keep the .rst filenames. And step 2. seems pretty trivial.

What do you think?

Start in mappers/base.py by tracing template loading and rendering, then inspect the existing configuration for the output suffix. Define how separate RST and Markdown template sets should be selected, convert the existing templates for Markdown, and replace hardcoded '*.rst' uses with configuration; done means Markdown tutorial and API documentation can be generated without breaking RST output.