Add docs

[hdrake]

This pull request introduces configuration improvements to support building documentation with Sphinx and Read the Docs. The main changes are the addition of a .readthedocs.yaml configuration file and updating the documentation dependencies.

Documentation build configuration:

• Added a .readthedocs.yaml file to configure Read the Docs, specifying the OS, Python version, Sphinx build settings, and dependency installation methods.

Documentation dependencies:

• Added sphinx to the docs/environment.yml dependencies to ensure Sphinx is available for documentation builds.

[hdrake]

@anthony-meza and @NelsonPoumaere, I drafted lightweight docs with just an installation page and the tutorial notebook. You can view the latest build here:

https://xbudget--23.org.readthedocs.build/en/23/

I'll update the homepage later today. Anything we should add?

[NelsonPoumaere]

That looks great @hdrake, and I don't see anything to add at the moment.