Change Documentation to documentation for QA
We no longer use ~Documentation (with capital D), it's actually documentation now (lower case d).
Couldn't figure out for months why the QA triage lists were including some docs changes, but not others. Turns out many were excluded due to using ~backstage or development guidelines.
Hopefully this will mean that anything with documentation, but not other labels like ~feature or ~bug (so documentation only MRs), will get excluded from the triage lists.
Related to https://gitlab.com/gitlab-org/release-tools/-/issues/315