Incremental improvements to custom types guide (inc. DOC-74)

Wow this page needs some work. This is a fraction of it.

- Explain namevar requirements and how to designate namevar
- Indent Ruby code with 2-space indents
- Give a bit more context
- Update waaayyy out-of-date restructured text reference
- Explain whitespace stripping in doc strings
Make the other PDL thing match what we did for PE3
The module publishing pages shouldn't use the legacy layout
I don't know why the quick nav was doing this, but it shouldn't.