Add modified core docs to website, Partially flatten navbar

[Kajmany]

Description

Ref #211 because I saw a maintainer was O.K with adding core to docs, but I also did some stuff you might not want along with that.

• Navbar flattening: When I first opened the docs I didn't realize there was more than two sections because we start inside of a nested menu rather than the root. I noticed the docs site for the docs site does the same thing, but I found it really confusing. I also thought the nesting in Tutorials and Release Notes were unnecessary. The navbar takes an awkward and laggy double-click to expand for me, so I think it is more convenient now.
• Adding core: Adding a blank __init__.py to core made this doable with an easy kludge inside the reference script, but I don't know if this causes problems elsewhere. I think this SO link might apply to why I had trouble getting mkdocstrings to load the modules -- it's purposeful by griffe? I'm not great with Python or Python packaging, so hopefully you know if it's a bad idea making core not be an implicit namespace package anymore(?)
• Fixing griffe warnings: The vast majority of these were type declarations that were redundant or had less information than the type hints in the functions themselves. A couple spots look like there was a refactor and the doc strings lagged behind. They don't look fantastic still: the Sphinx-style cross-references seem broken and some things like empty :rtype seem to make empty tables, but I hope it's good enough for now.

Tested locally with

rm -rf ./docs/reference/
uv run python scripts/mkdocs_generate_reference.py
uv run python scripts/mkdocs_generate_release_notes.py
uv run mkdocs serve

because it seemed like a lot of effort to replicate the GH-pages on a fork.

Also FYI that this doc generator is apparently being killed maintenance moded in favor of an open-source but commercial project. I would not be in a rush to migrate if I were you, though 🤷

Pre-merge Checklist

• I have described my change in the section above.
• I have ran the ./scripts/format.sh and ./scripts/lint.sh scripts. My code is properly formatted and has no linting errors.
• I have ran uv run pytest and ensured all tests pass no more fail than when I started.
• I have added my change to CHANGELOG.md under the [Unreleased] section.