Docs: add new cache persistence layer to Idempotency page

[dreamorosi]

What were you searching in the docs?

In #3896 we have added support for a new cache persistence layer in Idempotency. This allows customers to use a Valkey- or Redis OSS-compatible cache as persistence layer for the utility.

Is this related to an existing documentation section?

https://docs.powertools.aws.dev/lambda/typescript/latest/features/idempotency/

How can we improve?

The PR linked above already includes docs for the API reference, but we should update our documentation to have a similar structure to the one that the Python implementation has here.

Got a suggestion in mind?

No response

Acknowledgment

• I understand the final update might be different from my proposed suggestion, or refused.

[arnabrahman]

Please assign me on this

[dreamorosi]

We should also update this code snippet here to use a timeout - as recommended by the ElastiCache team here.