Update link for InvenioRDM documentation

[Herrner]

The link was dead. We could either link to the proposed install page, or remove that link alltogether.

❤️ Thank you for your contribution!

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[fenekku]

Hey thanks for that! Do you mind if I add to this PR a fix for all the other dead links to the docs? Since I changed the docs, I feel responsible to clean that up. (otherwise I would have let you do it 😸 )

[fenekku]

This covers all remaining dead links to the InvenioRDM documentation site.