document use of `occ.preview:cleanup` (v31 upcoming)

[joshtrichards]

See PR nextcloud/server#48537

Existing approach documented at https://github.com/nextcloud/previewgenerator?tab=readme-ov-file#i-want-to-resetregenerate-all-previews

[tcitworld]

Docs PR at nextcloud/documentation#12486