docs: document Agent Canvas Model Router

[juanmichelini]

Summary

• Adds an Agent Canvas Model Router guide.
• Documents the Add Model Router template chooser: Max Score, Min Cost placeholder, and Custom.
• Explains the default Pareto profile-generation flow and links to LLM Profiles, Agent Profiles, and SDK routing/profile docs.

Related implementation PRs:

• feat: add direct-prompt Model Router settings OpenHands#16147
• Add Pareto prompt meta-profile routing software-agent-sdk#4287

Validation

• python3 -m json.tool docs.json
• nav path existence check for every page in docs.json
• git diff --check

Note: mint is not installed locally, so I did not run mint broken-links. make llms-check shows broad existing generator drift outside this change, so this PR intentionally does not include generated llms.txt / llms-full.txt updates.

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[neubig]

Thanks for this! Could we confirm that a user (or an agent) is able to read these instructions and get it to work from scratch on a new installation of openhands. If not, we should make the documentation more concrete about how to do that.

[neubig]

Before merging this documentation, we should implement the provider connection feature first (OpenHands/agent-canvas#1657 / OSS-5295), and then follow this documentation flow:

1. Add the provider — examples with OpenHands and OpenRouter
2. Add the router — reference the issue above
3. Demonstrate that it works

Both the provider connection and the router need to be confirmed to work end-to-end from a fresh installation.

This comment was created by an AI agent (OpenHands) on behalf of @gneubig.