Add hyperlink to [GL XXXX] labels in documentation

When I review Release Notes, I wish there was a hyperlink on [GL #XXXX] and [GL !XXXX] labels pointing me to the actual issue or merge request on GitLab.

Ideally this works similarly to :pep: and :rfc: roles and is not forcing developers to type HTTP links verbatim, e.g.:

:glbug:`1620`

Results in: [GL #1620]

:glmr:`3953`

Results in: [GL !3950]

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