Use RTD for pull request previews

[plaindocs]

Hi folks.

It should be fairly easy to set up Read the Docs for pull request previews for the coding guidelines. I'm happy to configure that on my own fork and then make a pull request here if that's of interest?

[PLeVasseur]

This sounds like a very useful quality-of-life improvement to me!

[plaindocs]

OK, there is a pull request (against my fork) here which demonstrates the PR build, with a list of what needs to happen for this to go live.

plaindocs#1

[PLeVasseur]

Closed via #41

Thanks again for the suggestion @plaindocs!