Add initial files needed to setup Read the Docs

[allyhawkins]

This PR adds the initial files for starting the docs using Read the docs and begins the addition of some processing information. I started by creating the repo and following the quick start instructions using Sphinx with read the docs. This initiated the directory structure and filled out the initial files that are needed for read the docs. I also added in the MyST extension and followed the Using Markdown with Sphinx instructions to enable use of markdown files.

The only changes that were made to the files during the quickstart process were inserting the project name (scpca) and the author (The Childhood Cancer Data Lab).

[allyhawkins]

Can we ignore the _build directory? If so, I think we should add that to .gitignore (Remove the files first, to avoid annoyance). This will make future changes much smaller and easier to track.

Yes, I can add a gitignore file and put that in. I wasn't sure what to ignore since technically the -build directory is where the index.html lives which I assumed we needed?

We probably will want to add a requirements.txt file and perhaps a .readthedocs.yaml configuration file, but I expect that can happen later and doesn't much matter for now.

I also wasn't quite sure about these since I noticed they were in the refinebio-docs repo, but wasn't sure at what point we would need them. I can add those so we have them to start with? I didn't add any additional docs at this stage that aren't already pre-set during the quickstart but I can.

[jashapiro]

Yes, I can add a gitignore file and put that in. I wasn't sure what to ignore since technically the -build directory is where the index.html lives which I assumed we needed?

I'm 99% sure that this gets generated automatically: the _build directory should only be for local builds to verify how things look before it goes to RTD. Which is to say that I don't think we need this index.html: the index.rst file is the only one we would edit.

We probably will want to add a requirements.txt file and perhaps a .readthedocs.yaml configuration file, but I expect that can happen later and doesn't much matter for now.

I also wasn't quite sure about these since I noticed they were in the refinebio-docs repo, but wasn't sure at what point we would need them. I can add those so we have them to start with? I didn't add any additional docs at this stage that aren't already pre-set during the quickstart but I can.

I think the main value of these is so that anything we render locally works and looks the same as on RTD.