Change default Sphinx docs from reStructuredText to Markdown

[Spacecow99]

Considering our primary document hosting sources (Github, Github Pages and ReadTheDocs) all support Markdown in some shape or form, it is worth migrating our default sphinx documents over to Markdown by default and using the MyST package.

• Rename files from *.rst to *.md
• Reformat index.rst in to markdown
• Add myst-parser as a docs dependency in pyproject.toml
• Configure myst-parser and exclude .venv in conf.py

[Spacecow99]

Beta version using myst-parser in branch feature/myst-markdown-conversion. Will merge when testing complete.

No need to exclude venv patterns in conf.py, was just an error on my part during initial experimentation.

[Spacecow99]

One thing that should be noted with this is that myst-parser still expects docstrings to be written in reStructuredText when using sphinx.ext.autodoc as detailed here. This is a myst-parser limitation outlined in Github issue #228.