[docs] Add note on how to access generated Kibana encryptionKeys

[kilfoyle]

This updates the Advanced configuration page with a tip about how to access the operator-generated encryption keys.

Closes: #8129

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[stefnestor]

LGTM

[kilfoyle]

@elasticmachine run docs-build

[stefnestor]

I realized after testing that the ECK operator does not automatically set the decryptionKey for users, so added in addendums so if users are looking manually set these values but have previously used Kibana that they need to set the decryptionKey manually before rotating it.

[kilfoyle]

Thanks @pebrc!

[thbkrkr]

This command fails with error: template format specified but no template given. An = is important to assign the jsonpath.

Suggested change

	kubectl get secret my-kibana-kb-config -o jsonpath '{ .data.kibana\.yml }' | base64 --decode | grep -B 1 encryptionKey
	kubectl get secret my-kibana-kb-config -o jsonpath='{ .data.kibana\.yml }' | base64 --decode | grep -B 1 encryptionKey

Now it works, I feel that it can be confusing because there are several encryptionKey.

> kubectl get secret test-kb-config -o jsonpath='{ .data.kibana\.yml }' | base64 --decode | grep -B 1 encryptionKey
    encryptedSavedObjects:
        encryptionKey: K8MYAwOohLrviRp4tXlQqS7Gb7AWPe6DsRI9TUs0PoyHAdmpKDjupE8Q3ms4lwmQ
--
    reporting:
        encryptionKey: EcZHDgOz5BJqLdp26t6ittWg3tCJHkCIraciRUqDU2NncYbpO5DNJ4pt1Ayb5VEV
--
                        realm: oidc1
        encryptionKey: PIhbYdOg76JfzhnhzNpTz2JOasEj7oh1hpmZ47ZMEeTRR7Un1akRHVmA0pnDEBsw

I guess you don't want to provide an example using yq?

Then here is a proposal to continue using grep:

> kubectl get secret c14-kb-config -o jsonpath='{ .data.kibana\.yml }' | base64 --decode | grep -A1 encryptedSavedObjects
    encryptedSavedObjects:
        encryptionKey: K8MYAwOohLrviRp4tXlQqS7Gb7AWPe6DsRI9TUs0PoyHAdmpKDjupE8Q3ms4lwmQ

[kilfoyle]

Thanks @thbkrkr!
@stefnestor Please let me know if you like the proposal.

(I've added it in already but if you disagree, please let us know)

[kilfoyle]

@thbkrkr I see that Stef will be offline for a couple of days, so I think it's safe to merge this. If she'd like anything changed I can open a new PR.