Added documentation for sentry nodes deployment

[lucassaldanha]

Pull request checklist

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Make sure that:

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Describe the change

Added documentation for experimental sentry node feature.

Issue fixed

N/A

Impacted parts

• Documentation content
• Documentation page organization

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• You've fixed any issues raised by the tests.
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Preview

How To https://pegasys-teku--397.com.readthedocs.build/en/397/HowTo/Sentry-Nodes/

CLI https://pegasys-teku--397.com.readthedocs.build/en/397/Reference/CLI/Subcommands/Validator-Client/#xsentry-config-file

[CLAassistant]

All committers have signed the CLA.

[lucassaldanha]

Keeping this as a draft for now because I am not sure we want to document an experimental feature into the main doc.

[bgravenorst]

LGTM