Add a reference guide

[arporter]

With more people contributing/using fparser I think a Reference Guide would be useful. (At least, I'll certainly find it useful :-) ) This will be a two-stage thing - add the necessary files to build it and then create a new RTD project.

[arporter]

I need to update the project dependencies but the specification of these is moved by #327 so I need to wait until that goes on.

There are lots of errors and warnings generated from the existing docstrings. However, a lot of these are from fparser.one which we will be removing.