Update README and enable readthedocs

[helen-m-lin]

Is your feature request related to a problem? Please describe.
As a developer, I want to use an updated README so that I can easily setup and contribute to the repo.

Describe the solution you'd like

• Add Contribution guidelines, how to run tests/linters, and instructions for running or testing jobs locally.
  • E.g. you must start a ssh session locally and set host to 'localhost' in the JobSettings.
• Add description and example usage for the new populate_s3 and indexer jobs.
• Remove legacy jobs from readme
• Enable readthedocs

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[helen-m-lin]

Enable readthedocs:

• add readthedocs yaml
• create readthedocs account
• add readthedocs/sphinx files
  Also add mini-description to Readme.