merge pages of contribution guidelines

[axel-h]

Follow-up from the remark form @lsf37 in today's hangout call. The pages of with the contribution guidelines could be merged into one page. I've just stumbled over these pages

• processes/contributing.md
• processes/doc-contributing.md
  but there might be be even more. Goal is to have one singe page that people can be pointed to then.

[lsf37]

Just to clarify, merging everything into one page is not what I was after.

The point was more to have a single page with the most important information when you make a PR. There is way too much content to have all the relevant information on that page, and I do not want to remove that content. It is fine for this page to have links to more info, but it should have the essentials on it and be short enough that it can feasibly be read even for a small contribution. We sort-of do have a pull-request page, but it is lacking some context as a standalone page and probably is too long already.