Mention other ways to set encryption key for saved objects

[dedemorton]

Describe the feature:

The docs about setting an encryption key for saved objects should mention other ways to set the key (besides setting xpack.encryptedSavedObjects.encryptionKey in kibana.yml).

From what I understand, you can use the kibana-encryption-keys command, or Docker environment variables.

I've not personally used all these ways, so def confirm with the dev team.

Describe a specific use case for the feature:
Users who follow the UI link added here need to know there are other ways to set the encryption key.

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)

[watson]

The kibana-encryption-keys command is just for generating the encryption keys. When running the command the keys will be printed to the terminal and it's up to the person running the command to enter the keys into kibana.yml. I.e. this is not a different way of setting the keys, just a handy tool to generate them automatically.

But you're right that we of course should mention this tool on the "Secure saved objects" page, which we do not do currently.

For Docker you can set environment variables like:

XPACK_ENCRYPTEDSAVEDOBJECTS_ENCRYPTIONKEY="min-32-byte-long-NEW-encryption-key"
XPACK_ENCRYPTEDSAVEDOBJECTS_KEYROTATION_DECRYPTIONONLYKEYS[0]="min-32-byte-long-OLD#1-encryption-key"
XPACK_ENCRYPTEDSAVEDOBJECTS_KEYROTATION_DECRYPTIONONLYKEYS[1]="min-32-byte-long-OLD#2-encryption-key"

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)