docs: clarify plugin installation for in-cluster deployments

[iashutoshyadav]

This PR clarifies the recommended plugin installation approaches for
In-cluster Headlamp deployments.

It documents InitContainer + shared volume as a recommended option for
declarative and GitOps-based setups, and clarifies the role of the
sidecar-based plugin manager as an alternative supported mechanism.

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• ✅ login: iashutoshyadav / name: Ashutosh Yadav (dd8a6fc, f9a5ed4)

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[illume]

The plugin manager is intended mainly for development/testing.

This is not true.

[illume]

There's a lot of unrelated things removed in this PR.

I think it's also a good idea to wait until there is a consensus decision made in the issue first by the maintainers.