Fixes#30476
Minor tweaks to host identity cert renewal feature: added IANA number
and jitter
Mostly doc changes
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## Summary by CodeRabbit
* **Documentation**
* Added comprehensive documentation on managing custom OID extensions,
including structure, usage examples, and security considerations.
* Updated certificate renewal documentation to clarify timing details
and corrected the OID value to match Fleet’s assigned namespace.
* **Bug Fixes**
* Corrected the OID used for certificate renewal extensions to the
proper Fleet-assigned value.
* **New Features**
* Introduced random jitter (up to 30 minutes) to certificate renewal
timing to reduce simultaneous renewal requests.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Victor Lyuboslavsky
b2636d86ba