[DMF] add additional terms to "Credential Type"

[init-xbildung]

XBildung A comment from a german perpective, regarding the creadential types (as discussed in the XBildung-ELM-Workshop 27/03/2023)

Suggestion to add additional terms to "Credential type".

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The following types of credentials do exist (in addition) in the German education domain among others and are likely to play a role in the SDG implementation:

German Term	Suggestion in English from XBD
Schulzeugnis	school certificate
Sprachzertifikat	language proficiency certificate
Schulbescheinigung	certificate of enrolment in secondary education institution
Immatrikulationsbescheinigung	certificate of enrolment in tertiary education institution
Hochschulabschlusszeugnis	tertiary education completion certificate
Transcript of Records	transcript of records
Praktikumsbescheinigung	letter of confirmation for internship