Docs site split into Product and API sections

The /docs site is now split into two sections: Product (guides, agents, connectors, account, organization) and API reference. Switch between them from the dropdown at the top of the sidebar.

The bare /docs link now takes you straight to the Product guide instead of a page that no longer exists. The docs nav also gained a direct Changelog link, and the "Documentation" link in the dashboard sidebar now opens /docs/product.

Nothing in the guides or API reference content itself changed. This is a navigation update only.