Allow to query by token in @querysources API endpoint.

[deiferni]

With this PR we allow the @querysources API endpoint to be queried by a token parameter in addition to the existing query parameter. The token can be used to retrieve the representation of one already known token. It will return an empty list if no such token exists.

• I have added some very basic docs for our customizations to vocabularies and sources as specified in https://plonerestapi.readthedocs.io/en/latest/vocabularies.html
• I mostly used the description i found in REST API Schema-Intent Konzept in opengever.core umsetzen  #5973 and REST API: Schemas in Add- und Edit-Szenarien verfügbar machen #5968 for this
• I have amended tests so they also cover the possible error-states (no or too many query string parameters provided)
• I have also update the schema docs. this is a leftover of Fix empty bytestring overriding unicode in some cases. Fix existing content. #6601 and a should have done it there 😓

Jira: https://4teamwork.atlassian.net/browse/PHX-10

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• Changelog entry
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[lukasgraf]

LGTM 👍
Thanks for the docs regarding the schema intent! 🙈 🎉