upgrade v14: add instructions for updating vocabularies in v14

[tmorrell]

❤️ Thank you for your contribution!

Description

Add upgrade instructions for vocabularies

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've targeted the master branch.
• If this documentation change impacts the current release of InvenioRDM, I will backport it to the production branch following approval or indicate to a maintainer that it should be backported.

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[fenekku]

little enhancements, but good!

[tmorrell]

Thanks!