FIREFLY-1440: Set up continuous deployment for documentation website

[jaladh-singhal]

Fixes FIREFLY-1440

• Updated outdated sphinx docs infrastructure (Makefile, conf.py, etc.) using general Sphinx setup (over LSST specific one)
• Restructured docs/ content to make hierarchy clear
• Added a developer guide section in docs
• Created a github action workflow to build and deploy docs to github pages each time a commit is pushed to master

Testing

Checkout this branch locally. Update/create environment with the requirements.txt and make sure to install firefly_client. Follow the steps in docs/development/guide.rst to build and check the documentation.

Note: We will be able to test workflow and see the github pages at https://caltech-ipac.github.io/firefly_client/ only after this PR is merged. Which means this may require few iterations and/or direct pushes