[MRG] Replace most RST with Markdown

[asoplata]

This replaces the reStructuredText (RST) versions of the README and most of the hand-written RST content on the code website ( https://jonescompneurolab.github.io/hnn-core/stable/index.html ) with similar versions using MyST Markdown ( https://www.sphinx-doc.org/en/master/usage/markdown.html#markdown ). For the reasons why this is a good idea, see the point of discussion in #969. Note that the only truly new text I have added is a small part in contributing.md which explains our use of Sphinx/etc. Roles, including versions for both RST and MD: https://github.com/asoplata/hnn-core/blob/mardownify-code-website/doc/contributing.md#writing-the-documentation . I think this will make it easier for people to contribute to the documentation, particularly for new users who likely will never know RST.

[asoplata]

Please note that for the vast, vast majority of content (like, except for <30 lines), the HTML output of these Markdown-parsed files should be exactly the same as the current RST-parsed files. With the exception of about 30 lines of text-content (that is relevant to using MD vs RST), I have not made any changes to the existing text, but instead only converted it.

[asoplata]

The only real "actual content" changes are here https://github.com/jonescompneurolab/hnn-core/pull/973/files#diff-ed8d59a5653a55110f94eeaed1a1dd3323a630b040dab3afaf4d6ef91b1cca84R190-R227 . All other changes are negligible, such as replacing the full-link to github issues in https://github.com/jonescompneurolab/hnn-core/pull/973/files#diff-5bfc69eca3a4a9bd8b35373ce858c9572aed84a4e0d29cefa291c74d72d3a66dR87-R104 with usage of the local gh-substitution sphinx extension, or typo and code linkage fixes in whats_new.md.