website: rework Contribute page to avoid duplication with manual

The manual contains details about how users can contribute to
buildroot. The website also had such a list, but with much less detail and
not containing all topics.
In order to avoid duplicating this information, simply list the different
topics without detail on the website, and add a link to the relevent
chapter of the manual.

[Peter: slightly reword]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>