docs: Updating Gossip EncryptionKey Rotation page with Vault use case (#12720)

* docs: Updating Gossip EncryptionKey Rotation page with Vault use case

* Adding a note to the vault instructions linking to the gossip key encryption using Vault page.

* Correcting Vault guide for storing the rotated gossip key.

* adding $ to shell sessions where it is missing on the gossip rotation page

* adding $ to more shell sessions where it is missing on the gossip rotation page