CONTRIBUTING.md: Misc improvements

- In general, many sections have been expanded a lot.
- Defer to the wiki when it makes sense.
- Rewrite a bunch of things to be clearer. There is a lot more
  verbosity, but clarifying as many items as we can in this document is
  preferable to it being done later in the issue tracker.

(alphabetically sorted)
Helped-by: David Granström <david@davidgranstrom.com>
Helped-by: Florian Walch <florian@fwalch.com>
Helped-by: John Szakmeister <john@szakmeister.net>
Helped-by: Justin M. Keyes <justinkz@gmail.com>
Helped-by: Martin Kopischke <martin@kopischke.net>
Helped-by: oni-link <knil.ino@gmail.com>

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