Remove Nebari-HPC docs

[pavithraes]

Nebari-HPC docs were moved during the QHub->Nebari migration, but we don't actually have a Nebari-HPC project to share right now.

This PR removed the Nebari-HPC docs and all mentions of HPC support.

We can add these doc back and update them, when there is a stable user workflow for HPC deployments :)

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[pavithraes]

@viniciusdc Would you mind taking a look at this when you get a chance? Thanks!

[viniciusdc]

LGTM, thanks @pavithraes

[viniciusdc]

Once this is merged we will follow up the work on #435