Update the contributing documentation link on README

What does this MR do?

The dev docs was recently revamped and moved to the gitlab documentations site (#730 (closed)), which means that now we can point to the new version of that documentation in the readme and delete the old one.

Related to: #774 (closed)

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines