Switch docs to Zensical

[waylan]

Description

The following changes have been made.

• Remove dependency on mkdocs-gen-files, which is not currently supported by Zensical. The Markdown files are no longer auto-generated on each build, but permanently in the docs source tree.
• The mkdocs.yml config file and custom css has been altered to use and support Zensical's modern theme.
• The Library Reference page has been rewritten as prose with more code examples (we have generated API docs, so the usage docs should not also be formatted as API docs). The file has also been renamed from reference.md to library.md to avoid conflicts with the API docs which are at reference/.
• Various minor formatting edits have been made to the docs to better fit with the new theme.

AI Assistance Disclosure

• No AI tools were used in preparing this PR.
• If AI tools were used, I have disclosed which ones, and fully reviewed and verified their output.

Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
• I have added or updated relevant docs, including release notes if applicable which follow the [Documentation Style Guide](Documentation Style Guide).
• I have added or updated relevant tests.
• I have not requested, and will not request, an automated AI review for this PR.

[waylan]

Hmm, getting a weird error during build that I can't replicate locally. Will need to explore what is causing it.

[waylan]

Here are some screenshots. Some pages are scrolled to show different features.

[waylan]

We also get a dark mode.