Add a new `Mobile Apps` or `Clients` Category

We already have pages for mobile apps (ex: https://apps.nextcloud.com/apps/android_nextcloud_app) and should have a category that shows those.

No files or tests are named. Start by finding how existing app categories are defined and how the Android app listing is classified; then determine how a Mobile Apps or Clients category should select those listings. Done means the category exists and shows the relevant mobile app pages.

Written by the indexing model from the issue text.