Encode types of correspondence medium: postcard, telegram

[StefanDumont]

Thomas Kollatz, among others, suggested at the meeting of the TEI Correspondence SIG on 6.9.2023 in Paderborn that a classification of correspondence types could be developed (especially for the CMIF). Correspondence types are to be understood here in a narrower sense and are aimed more at the form / medium, i.e. telegram, postcard, etc.