Rename category "Use Case" to "use-case" to remove whitespace

See https://gitlab.hzdr.de/hifis/overall/hifis.net/-/merge_requests/511#note_1509020

assigned to @frust45

mentioned in merge request !511 (merged)

created branch 336-rename-category-use-case-to-use-case-to-remove-whitespace to address this issue

mentioned in merge request !552 (merged)

mentioned in commit 2feab6d3

closed with merge request !552 (merged)