Add FAQ entry on keystore passwords

[SgtCoDFish]

This has been a hotly requested thing when we've been talking about these passwords for both cert-manager and trust-manager, and it was discussed in the biweekly meeting on 2023-11-30.

Longer term, this FAQ entry should provide us with justification for allowing users to specify these "passwords" in plaintext in their Certificate resources, replacing passwordSecretRef on JKSKeystore and PKCS12Keystore.

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[inteon]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

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[erikgb]

A huge LGTM from me also. Thanks Ash! I don't have reviewer permissions on this project yet, but I'm planning to create a PR for that.

[SgtCoDFish]

/unhold

Thanks both!