Refresh design documents and expose in rtd

[ethanfrey]

Is your feature request related to a problem? Please describe.

https://weave.readthedocs.io/en/latest/index_old.html#weave-architecture gives an overview of the high-level design. However, this has not need refreshed for the latest 0.17 release and not ported over to the main index.rst file.

This is essential information to understand weave and should be easy to find.

Describe the solution you'd like

docs/design should be updated
index.rst should include all essential points
Add other sections as needed, such as orm, migrations, ???