docs: document Agent Canvas Model Router - #687

Running Vale across on the following files affected by this PR:

- openhands/usage/agent-canvas/llm-profiles.mdx
- openhands/usage/agent-canvas/model-router.mdx

Attempting to initialize Vale linter...
No customer vale configuration detected. Using Mintlify's default configuration.
Attempting to run Vale spellcheck...
Vale spellcheck completed successfully.

openhands/usage/agent-canvas/llm-profiles.mdx:
	- 45:34     Did you really mean 'Ollama'?
	- 45:48     Did you really mean 'SGLang'?
	- 55:67     Did you really mean 'Ollama'?
	- 98:107    Did you really mean 'reachability'?

openhands/usage/agent-canvas/model-router.mdx:
	- 19:39    Did you really mean 'prefilled'?
	- 20:38    Did you really mean 'prefilled'?


Found 6 suggested changes across 2 files in 806.64ms