docs: cover Agent Canvas 1.2.0 updates

[DevinVinson]

Summary

• add Agent Canvas docs pages for plugins, conversations, and Agent Profiles
• update Agent Canvas overview/onboarding/settings/LLM profile docs for the 1.2.0 release surfaces
• document command palette discovery and automation LLM profile editing

Closes #613

Validation

• Touched-file internal link check passed
• uv run --with pytest --with requests --with pyyaml pytest -q tests/ reached 34 passed, 2 errors from existing pricing tests because https://raw.githubusercontent.com/OpenHands/OpenHands/main/openhands/utils/llm.py returned 404
• Ran make llms, but did not include generated llms.txt / llms-full.txt drift because it included broad unrelated repository changes

[enyst]

Thank you!