Added line to pre reqs for creating DLP key to use prompt logging

Using AI Prompt logging requires a DLP public key to be created before Gateway will log prompts. Added this step to the prereqs to prevent confusion when AI prompts aren't logged due to no DLP key existing.

Author: dledfordcf

src/content/docs/cloudflare-one/policies/gateway/http-policies/granular-controls.mdx

@@ -17,6 +17,7 @@ To use Application Granular Controls, you must:
 - Turn on [TLS decryption](/cloudflare-one/policies/gateway/http-policies/tls-decryption/).
 - Turn on the [Gateway proxy](/cloudflare-one/policies/gateway/proxy/#turn-on-the-gateway-proxy).
 - (Optional) If an application uses HTTP/3, turn on the [Gateway proxy for UDP traffic](/cloudflare-one/policies/gateway/http-policies/http3/#enable-http3-inspection).
+- (Optional) If you wish to enable AI Prompt logging, create a [DLP payload encryption public key](https://developers.cloudflare.com/cloudflare-one/policies/data-loss-prevention/dlp-policies/logging-options/#set-a-dlp-payload-encryption-public-key).
 
 ## Create a policy with Application Granular Controls