Add note on Gateway API Manual Deployment and Policy Attachment

[dgn]

Description

Users will have to add this label for Policy Attachment to work properly.

Reviewers

• [ ] Ambient
• [ ] Docs
• [ ] Installation
• [x] Networking
• [ ] Performance and Scalability
• [ ] Extensions and Telemetry
• [ ] Security
• [ ] Test and Release
• [ ] User Experience
• [ ] Developer Infrastructure
• [ ] Localization/Translation