Restructuring and cross-linking of sasmodels docs

[smk78]

This PR addresses all the comments in:

• https://github.com/SasView/sasmodels/issues/397
• https://github.com/SasView/sasmodels/issues/387
• https://github.com/SasView/sasmodels/issues/205#issuecomment-603207732

It also resequences the Contents into what I think is a more logical order?

[smk78]

I didn't look too closely at the text and the math. It should be checked by someone else anyway since it smells like my writing.

Yup! :-) Because you specifically asked for your comment to be added to the docs @pkienzle !