Add equation number to readthedocs

[rthedin]

When referring users to specific parts of the modeling guidances (for FAST.Farm for example), it's hard to point them to a specific equation since not all of them are numbered (example). Currently, only the equations that are referenced in the text gets a number. Maybe we should number all of them?

[andrew-platt]

That's a good idea! It may take a while to do.