docs: add session_child_first keybinding to documentation (#16631)

Author: ariane-emory

packages/web/src/content/docs/keybinds.mdx

@@ -28,6 +28,7 @@ OpenCode has a list of keybinds that you can customize through `tui.json`.
     "session_unshare": "none",
     "session_interrupt": "escape",
     "session_compact": "<leader>c",
+    "session_child_first": "<leader>down",
     "session_child_cycle": "<leader>right",
     "session_child_cycle_reverse": "<leader>left",
     "session_parent": "<leader>up",