Migrate documentation to mkdocs + various improvements

[davidwyld]

Resolves #112

• Migrates from sphinx to mkdocs-material
• Restructures the docs, reducing nesting etc.

There's still a fair bit to do here:

• Some repetition across the dataloaders, e.g. similar content in scalar, vector and LUT dataloader pages which could be consolidated,
• There's probably more we can do with automatic documentation from docstrings, particularly with the dataloaders (also see Review dataloaders #113) - there's a lot of extra information in the docs for each dataloader, some of which I like, but could maybe be kept in the dataloader docstrings themselves and be rendered in the autoapi docs?
• Quite a lot of typos and spelling errors throughout the docstrings which we could sort out.
• In many places the docs are a bit verbose for my tastes, though this is personal preference.
• There's a lot of documentation, which is great! But perhaps I could tidy it up a bit more in places.
• There are a lot of warnings from mkdocstrings/autoapi right now but when Adding typehints #108 is merged in, hopefully most will be resolved.

Note: Merge target is set to github-pages-docs for now to keep the diff relevant, but we should really get that branch merged soon and this can target main instead.

[thomaszwagerman]

Things you love to see:

• it

[thomaszwagerman]

Looks great. Quite a few warnings from griffe, but as you say, we can tackle those and they aren't necessarily to do with the docs themselves, rather the docstrings.