Add docs for the specific benefits of using the Pinniped Supervisor

[cfryanr]

Is your feature request related to a problem? Please describe.

This would answer a very common question. The docs do not make the benefits clear enough. E.g. see this conversation https://kubernetes.slack.com/archives/C01BW364RJA/p1674080319214249?thread_ts=1673968325.113179&cid=C01BW364RJA

Describe the solution you'd like

A new doc on the public web site.

Describe alternatives you've considered

None.

Are you considering submitting a PR for this feature?

Yes. This is docs only, so no test or architectural changes.

• How will this project improvement be tested?
• How does this change the current architecture?
• How will this change be backwards compatible?
• How will this feature be documented?

Additional context