Remove redundant names in OpenPGP subtree

[daringer]

e.g., this article: https://docs.nitrokey.com/nitrokeys/features/openpgp-card/openpgp-thunderbird

is called "openpgp-thunderbird" within the category "openpgp" - instead it should just be "thunderbird". this includes the titles, which should also change the navigation links and also the filenames should be adapted accordingly - think I saw 3 similar occurences