Doc: Secure boot V2: Revoke previously unused keys if they are comprimised

[Lupindakaas]

Description

This PR cleares up a unclear part of the secureboot v2 documentation. The current documentation fails to mention that any key can be revoked, not just key N-1. This PR adds to the existing documentation by describing when and how you should revoke a not currently unused key.

Related

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

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Checklist

Before submitting a Pull Request, please ensure the following:

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

[github-actions]

Warnings

⚠️

Some issues found for the commit messages in this PR: the commit message "Capitalization fix": summary looks empty type/action looks empty the commit message "Update secure-boot-v2.rst": summary looks empty type/action looks empty Please fix these commit messages - here are some basic tips: follow Conventional Commits style correct format of commit message should be: <type/action>(<scope/component>): <summary>, for example fix(esp32): Fixed startup timeout issue allowed types are: change,ci,docs,feat,fix,refactor,remove,revert,test sufficiently descriptive message summary should be between 20 to 72 characters and start with upper case letter avoid Jira references in commit messages (unavailable/irrelevant for our customers) TIP: Install pre-commit hooks and run this check when committing (uses the Conventional Precommit Linter).

	Messages
📖	You might consider squashing your 4 commits (simplifying branch history).

👋 Hello Lupindakaas, we appreciate your contribution to this project!

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