Use conda-sphinx-theme (plus redirect fixes)

[travishathaway]

Description

Fixes: #374

This also re-applies changes that were previously reverted because the redirects were broken.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation

How to build locally

Here are instructions for building the documentation site locally. Run these commands from the root of the project directory:

cd docs
conda env create -f environment.yml
conda activate conda-libmamba-solver-docs
make html

[jaimergp]

I'll merge this after cutting 23.11.1!

[jaimergp]

Thanks @travishathaway! Working now!