docs: document skipped agent definitions

[px12811]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

Document that a file-based or plugin agent definition which cannot be registered, such as one referencing an unknown skill, is skipped with a warning while other valid definitions continue to load.

This is the documentation counterpart to OpenHands/software-agent-sdk#5680.

Source Verification

• register_file_agents() and register_plugin_agents() now register through _register_agent_definition(), which catches ValueError, logs a warning, and continues: https://github.com/px12811/software-agent-sdk/blob/fix/skip-subagent-missing-skill/openhands-sdk/openhands/sdk/subagent/registry.py
• The corresponding SDK invariant is documented in the subagent guide: https://github.com/px12811/software-agent-sdk/blob/fix/skip-subagent-missing-skill/openhands-sdk/openhands/sdk/subagent/AGENTS.md