Update documentation for the `mutating webhook` and `Secret Init`

[csatib02]

Preflight Checklist

• I have searched the issue tracker for an issue that matches the one I want to file, without success.
• I agree to follow the Code of Conduct.

Problem Description

The Secrets Webhook project was created along with Secret Init. Their predecessor Vault Secrets Webhook and Vault Env will be deprecated, so the current documentation will be outdated.

Proposed Solution

Revisit the docs of the Mutating Webhhok and rewrite parts that was affected by the creation of the new projects.

Alternatives Considered

No response

Additional Information

No response