[Gateway] MCP detection, AI Security dashboard, and Traffic Source selector changelogs

[grstnhbr]

Summary

This PR adds two Gateway changelog entries and updates HTTP policy documentation for Agents Week:

1. MCP protocol detection and AI Security dashboard

• Documents the new experimental.is_mcp selector in HTTP policies that detects Model Context Protocol traffic
• Introduces the AI Security dashboard under Insights and Logs > Dashboards
• Adds the experimental selector note (beta, may change before GA, changes communicated before docs update)

2. Traffic Source selector in Gateway policies

• Documents the new Traffic Source (net.onramp.type) selector for HTTP and Network policies
• Enumerates all available on-ramp values: Device client, Mesh, Cloudflare WAN, Clientless RDP, Proxy endpoint, Clientless Browser Isolation, MCP portal
• Documents the Browser Isolation (net.is_isolated) selector

Documentation changes

• Adds experimental.is_mcp selector to HTTP policies reference page with beta note
• Updates the HTTP Allow/Block selector list partial to include the new selector

Note

The gateway-mcp-dashboard.png image is a placeholder and needs to be replaced with the actual screenshot before merge.

[cloudflare-docs-bot]

Review

✅ No issues found in commit 3a2be30.

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[grstnhbr]

Skipping the following review findings:

SG-1d0dd3ba, SG-8057c5d8, SG-33744a8c - The fragment-before-table pattern is consistent with every other selector on both pages (for example, "The HTTP request method used in the traffic." on line 664, "The pathname of a webpage's URL." on line 734, "The continent of the user making the request." on line 692). Changing just the new selectors to use a colon-terminated sentence would make them inconsistent with the rest of the file.