readme - rm broken link to rtd, point to cern

[phette23]

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Description

The readthedocs link is broken, it seems like most documentation has been moved to docs.cern.ch. There's two places for vocabs, one under Customize and one under REST API, but Customize seems more general. Not sure if there is a more generic (not RDM) set of docs out there.

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 added relevant test cases.
• I've added relevant documentation.
• I've marked translation strings (for relevant code).
• I've followed the CSS/JS and React guidelines (for relevant code).
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• I've followed the user interface guidelines (for relevant code).
• I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• I've NOT included third-party code (copy/pasted source code or new dependencies).

Third-party code

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