Improve development doc structure

[dstansby]

• Remove the roadmap, since this seems to be a historical document that is out of date. Alternatively we could keep this here and put a disclaimer that it's a historical document at the top?
• Split the developer's guide into two pages, one for contributors, and one for maintainers. I think this makes the content more discoverable (for both sets of people), and also keeps the contributor's guide a bit simpler.

[jhamman]

I'd like to keep the roadmap page. There is already a disclaimer explaining the status / historical significance of the page. Medium term, I'd like to update this document. The 3.0 release opens the door to some really cool stuff and this is the place where we can explain where the project intends to go.

[dstansby]

👍 - I didn't spot the note initially (my bad), I've just moved it to the top of the page in the latest revision.