Index set docs

[akilnarayan]

Write basic technical documentation for HyperbolicCrossSet and LpSet.

Make updates on the branch index-set-docs that is already pushed to github. That branch has the TotalDegreeSet docs implemented for reference. See UncertainSCI/indexing.py.

To locally compile the documentation to check your changes, see https://uncertainsci.readthedocs.io/en/latest/dev_docs/HowToTutorial.html#testing-locally. After you re-build the documentation locally, your changes will appear in your local copy of the page https://uncertainsci.readthedocs.io/en/latest/api_docs/spaces.html