Add configuration for mkdocs

[alexdewar]

This PR adds support for building documentation and auto-publishing it to GitHub Pages via a CI workflow. This includes both API documentation generated from the source code as well as technical documentation in .md files (at present, it just includes the files that were in the Tutorials folder).

To see the result:

1. Check out this branch
2. Install new dependencies with poetry install
3. Run mkdocs serve -o to build the documentation and view in a web browser.

The API documentation is generated with mkdocstrings-python, which supports a number of docstring styles (see here). At present, however the docstrings seem to be in a mixture of Google style and sphinx style, so the parsing is not great (i.e. of arguments etc.). Longer term, it would make sense to reformat the docstrings to use one style so that mkdocs can process them correctly. (When you build the documentation, you'll get a bunch of warnings from the underlying griffe library which should give you hints about what needs fixing.)

For now, I've set the CI job to run on every push to main, but at some point, you may only want to update the documentation on releases at which point you can tweak the config file. To configure your repo to publish to GitHub Actions via a workflow, you need to follow the steps here: https://mkdocstrings.github.io/griffe/reference/docstrings/

I had to bump the version of the regex package because the version specified in pyproject.toml was too old for the latest version of mkdocs.

Let me know if you have any questions!

Closes #30.

[Thomas-Rowlands]

Tested and looks good

[Thomas-Rowlands]

Merge conflict due to the previous PR merges for PyTest altering the dependencies, needs the poetry.lock file regenerating and a new .toml file with combined PyTest/mkdocs changes.

[alexdewar]

Done. Thanks for review!

[alexdewar]

Looks like I can't merge this myself, but it's ready to go now.

[AdrianDAlessandro]

This looks good to me! We should open an issue about the docstrings style inconsistency

[alexdewar]

Done: #49