Add nimbus VC

## Problem to be solved

Nimbus has added [instructions](https://nimbus.guide/validator-client.html) for a validator client (VC). See note in [this](https://docs.rocketpool.net/guides/node/install-modes.html#the-hybrid-configuration-with-external-clients) section.

## Proposed solution

Add nimbus as part of the docs in `docs.rocketpool.net/src/guides/node/install-modes.md`.