Improve BitBucket integration documentation [ci skip]

This MR will improve the BitBucket integration documentation by

- mentioning the need to reconfigure GitLab after changing the configuration file, if you're using the omnibus package.
- adding a section about configuring a Host entry for bitbucket.org.
- switching additional steps 1 and 2, because connecting to BitBucket will only work after you added the SSH key and the Host entry.
- using sudo for the ssh connection in step 2, so that it is more obvious, that the git account should be used to establish the connection.

See merge request !670