* docs: add subagents feature documentation
Add new documentation page covering the Subagents feature, including
how it works, enabling/configuring, auto-approve behavior, available
tools, and usage guidance. Register the page in docs.json sidebar nav.
* docs: remove hardcoded subagent limit from subagents page
Remove references to 'up to five' subagents, as the limit is no longer
fixed. Updates both the intro paragraph and the How It Works section.