Add instructions to run tests + build docs locally

[navidcy]

This PR adds a section in the docs to explain how contributors can:

• run the tests locally
• build and preview the docs locally

https://mom6-regional--44.org.readthedocs.build/en/44/contributing.html

[codecov]

Codecov Report

Merging #44 (5257230) into main (91fcc59) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #44   +/-   ##
=======================================
  Coverage   10.72%   10.72%           
=======================================
  Files           2        2           
  Lines         429      429           
=======================================
  Hits           46       46           
  Misses        383      383           

[navidcy]

I admit I didn't try the pip version. But the conda version worked for me!

[angus-g]

This won't work, would be just pip install -r requirements.txt since you changed into docs in the previous line.

[navidcy]

I'll fix it!