chore(docs): updates URLs and tagging instructions used in release proce...

...ss workflow

- The URLs were pointing to the old server
- Tagging instructions are now clearer
Were the old instructions faulty or did I just understand them wrong? How would the tag appear in ```1.9``` branch if I added it to ```release-1.9.0-rc.6``` branch?

BTW, I suppose most of us call Elgg/Elgg "upstream" instead of "origin". Should we change it here?