DOC: add links to other pages to contributing.rst
Awaiting for that the different developer pages could be reworked and added to this page, I thought to at least add some links here. So if people go from the docs to this page, they get redirected to the relevant pages.

@jorisvandenbossche do we have an open issue to incorporate the developer pages here (and then link back on pydata)? if not ...let's create one
 Not yet I think, will do now. Merging this.
