Merge branch 'alt-text-guidelines' into 'master'

Update alt text guidelines

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167754



Merged-by: Fiona Neill <fneill@gitlab.com>
Approved-by: Fiona Neill <fneill@gitlab.com>
Co-authored-by: Ashraf Khamis <akhamis@gitlab.com>