docs: document adding and updating gateway policies

[renuka-fernando]

Purpose

The gateway docs explain how to build a gateway with custom policies, but not how to add a managed policy, update a policy to a different version, or roll the change out. This PR adds a dedicated how-to and aligns the surrounding pages with the current ap CLI (v0.9.1).

Preview

Approach

• Add "Add or Update Policies in a Gateway" across all four gateway doc versions (1.0.0, 1.1.0, 1.2.0, next), wired into the nav and the un-versioned redirect.
• Cover adding managed (Policy Hub) and local policies, pinning to a major/minor/exact version, rebuilding the image with ap gateway image build, and rolling it out via docker-compose.
• Update the custom-policies build page for ap CLI v0.9.1: versioned and platform-neutral download, folder-aware extraction, and the ap version command.
• Split the AI Gateway guardrails extend section into "Add or update guardrails" (linking the new page) and "Custom guardrails" (linking "Writing a Custom Policy"), across all versions.

Related Issues

N/A

Checklist

• Tests added or updated (unit, integration, etc.)
• Samples updated (if applicable)

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