Configuration docs refresh

As @minrk pointed out in #4475, our config docs said a lot about how the config system worked, and not enough about how to use it. Here, I have:

- Moved the technical details of the config system into the development directory.
- Added a new 'intro' document describing how to use the config system, without the technical info.
- Autogenerate lists of config options for the main components (this doesn't yet include the parallel machinery).
- Condense and tidy up other config docs, removing outdated information.