Add node explanation to docs

"Your node. Your plan doesn't include a node \[Add your own node\]" => "Add node. A node is a machine where AI agents build and test code for you. \[Add Node\]" Add the above clarification of what node is to the docs and where user meets this for the first time

Setup step "Your node" is now "Add node". It says "A node is a machine where AI agents build and test code for you." and its button is now Add node. The same sentence is in the guided-setup and what-is-Jaah docs. Quick gate and 97 vitest green; visual check passed at desktop and phone widths. PR #6579 queued. Memory: none applied.