docs: Explain revocation of unused but compromised keys (IDFGH-13688)

[Lupindakaas]

Description

This PR updates the Secure Boot V2 documentation to explain the revocation of unused but compromised keys.

Related

https://docs.espressif.com/projects/esp-idf/en/v5.2.2/esp32s3/security/secure-boot-v2.html#multiple-keys

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Before submitting a Pull Request, please ensure the following:

• [ x] 🚨 This PR does not introduce breaking changes.
• [ x] All CI checks (GH Actions) pass.
• [ x] Documentation is updated as needed.
• [x ] Tests are updated or added as necessary.
• [ x] Code is well-commented, especially in complex areas.
• [x ] Git history is clean — commits are squashed to the minimum necessary.

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[AdityaHPatwardhan]

@Lupindakaas thanks for the PR.
I have some nitpicks but overall changes look good to me.

[AdityaHPatwardhan]

@Lupindakaas Thanks for your quick revert. LGTM
Can you please squash the commits into one?

[AdityaHPatwardhan]

sha=b5f0e5c6af639d41bee0c970bae982967bca08e9

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All committers have signed the CLA.

[Lupindakaas]

@AdityaHPatwardhan Thanks for the review! I have (through many trials and trebulations) succesfully squashed the commits. Could you merge this PR? Have a nice day!

[AdityaHPatwardhan]

sha=39d0eacde403051f8ecaa220f2cf4ca597c3f844