Community providers: Allow for installation files to require only _index.md

[guineveresaenger]

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Issue details

Currently, we require two installation instruction files for community providers, but internally, we do fine with just an _index.md as both landing page and install-config how-to.

The Registry already knows how to render providers with only the _index.md file. We should streamline this requirement to be the same for community packages.

Example: #6188 would fail today.

Affected area/feature

Community providers