Replace Development page with Contributing page

Since the _Development_ section is all about contributing (reporting bugs, submitting feature requests, submitting pull requests), renaming it to _Contributing_ makes sense.

See also #58.

To do:

* [ ] Set up redirects for old pages
* [x] Add intro paragraph, so that we don’t have two headers back to back
* [x] Include reviewing pull requests as a recommended way of contributing (possibly out of scope)
* [x] Include donations as a way of contributing (possibly out of scope)

Also [on staging](http://staging.nanoc.ws/_branches/feature-contributing-section/contributing/).