docs: How to recover from a situation when TPM looses keys

[samipietila]

Please describe the question or issue you're facing with "Use a recovery mode - Ubuntu Core documentation".

https://documentation.ubuntu.com/core/how-to-guides/manage-ubuntu-core/use-a-recovery-mode/

The documentation says that if TPM looses keys, the key from "sudo snap recovery --show-keys" can be used to boot the machine. That works. However, the documentation does not say what to do next, that is, how to return the key to TPM. Furthermore, apparently the snap recovery key is not luks key nor it is clear how to get luks key from snap recovery key.

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Reported from: https://documentation.ubuntu.com/core/how-to-guides/manage-ubuntu-core/use-a-recovery-mode/

[GairePravesh]

This section should also include /v2/system/system-recovery-keys endpoint as alternative to the sudo snap recovery --show-keys, people would prefer to fetch it via api in automated installations.