Add an FAQ on how to set Rudder Anonymous id as posthog identifier

[ranganathkoman]

What do these changes do?

Describe the changes you are proposing in this PR. FAQ to help set RudderStack anonymousId as posthog's unique identifier

Type of documentation

• [ ] Documentation update

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Name	Status	Preview	Comments	Updated (UTC)
rudderstack-docs	❌ Failed (Inspect)			Jul 24, 2023 3:09pm

[ameypv-rudder]

Incorporated the FAQ here. Closing the PR.