docs: fix broken contributing links
docs: fix broken contributing links

* Updates some of the links to the new invenio.readthedocs.org domain.

* Removes links that do not have a direct match in the new docs.
  (closes #3557)

Signed-off-by: Joao Goncalves <f54016ff1b253b48f02013d53a8a9385f3432a79@gmail.com>
inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio