Adding documentation using RTD and Sphinx

[surbhigoel77]

Fixes #49 #51

At present, we are able to build RTD for existing README.md locally (trying to build in on the RTD platform to make it live). Once the documentation pipeline is functioning, we can add new docs and pages.

Suggested content:

1. Why Spinup-Evaluation
2. Installation
3. Evaluation Quickstart
4. Benchmarking dataset guide
5. Command line scripts
6. Evaluation Metrics (Deterministic, Probabilistic, Physical-Metrics)

For Automation:

• .readthedocs.yml will automatically triggers a build upon a new commit.

[ma595]

@surbhigoel77 what's the current status of this PR?

[ma595]

Hi @SimonSadler I think this is the PR that Surbhi opened for the RTD work. I think it need a little tidy up before merging. Let's discuss this over Slack or in the internal meeting at some point.