Resolve "Unify usage of `boeking` vs `afspraak` on the platform"

What does this MR do?

This MR changes some of the text used for both the Dutch and English version.

I opted for "afspraak" over "boeking", as this felt more natural to me as we generally also say "een afspraak maken bij de dokter".

Does this MR meet the acceptance criteria?

• I have added tests to validate the new functionality.
• I have updated the documentation accordingly.
• I have added translations for all new client-side text.
• I have updated the changelog accordingly. Closes #181 (closed)