Add experimental warning banner to campaign documentation

[Copilot]

Campaign features need visibility into their experimental status. Revives banner from PR #12685 with updated messaging.

Changes

Added banner to campaign docs:

• guides/campaigns/index.mdx
• guides/campaigns/getting-started.md
• examples/campaigns.md

Banner

banner:
  content: '<strong>⚠️ Experimental:</strong> Campaign features are still experimental and subject to change.'

Renders as styled warning at top of each page in Starlight theme.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• telemetry.astro.build
  • Triggering command: /opt/hostedtoolcache/node/24.13.0/x64/bin/node node /home/REDACTED/work/gh-aw/gh-aw/docs/node_modules/.bin/astro build grep -l banner: orkflows-multi-phase.md ep js/**/*.json' --sh -json 64/bin/go grep -l banner: orkflows-interactive-chatops.md nfig/composer/vendor/bin/grep -m -json 64/bin/go grep (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Revive campaings banner from this PR: #12685
But make it mention that this feature is still experimental and subject to change (or simlar text) in banner.

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