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
• Networking
• Performance and Scalability
• Extensions and Telemetry
• Security
• Test and Release
• User Experience
• Developer Infrastructure
• Localization/Translation

[istio-policy-bot]

😊 Welcome! This is either your first contribution to the Istio documentation repo, or
it's been a while since you've been here. A few things you should know:

• You can learn about how we write and maintain documentation, our style guidelines,
  and the available web site features by visiting Contributing to the Docs.

• In the next few minutes, an automatic preview of your change will be built with
  a full copy of the istio.io website. You can find this preview by clicking on
  the Details link next to the deploy/netlify entry in the status section of this
  page.

• We care about quality, so we've put in place a number of checks to ensure our documentation
  is top-notch. We do spell checking, sanitize the Markdown, ensure all hyperlinks point to a
  valid location, and more. If your PR doesn't pass one of these checks, you'll see a red X in the
  lint_istio.io entry in the status section. Click on the Details link to get a list of the
  problems with your PR. Fix those problems and push an update; this will automatically re-run the
  tests. Hopefully this time everything will be perfect!

• Once your changes are accepted and merged into the repository, they will initially show up
  on https://preliminary.istio.io. The changes will be published to https://istio.io
  the next time we do a major release (which typically happens every 3 months or so).
  To publish them sooner, add a cherrypick/release-x.xx label, where x.xx is the current
  release of Istio.

Thanks for contributing!

Courtesy of your friendly welcome wagon.