Modes docs (#3233)

* Modes docs

* Added current mode

* fix docstring

* diagrams

* Update docs/guide/screens.md

Co-authored-by: Rodrigo Girão Serrão <5621605+rodrigogiraoserrao@users.noreply.github.com>

* Update docs/guide/screens.md

Co-authored-by: Rodrigo Girão Serrão <5621605+rodrigogiraoserrao@users.noreply.github.com>

* words

---------

Co-authored-by: Rodrigo Girão Serrão <5621605+rodrigogiraoserrao@users.noreply.github.com>

docs/examples/guide/screens/modes01.py

@@ -0,0 +1,42 @@
+from textual.app import App, ComposeResult
+from textual.screen import Screen
+from textual.widgets import Footer, Placeholder
+
+
+class DashboardScreen(Screen):
+    def compose(self) -> ComposeResult:
+        yield Placeholder("Dashboard Screen")
+        yield Footer()
+
+
+class SettingsScreen(Screen):
+    def compose(self) -> ComposeResult:
+        yield Placeholder("Settings Screen")
+        yield Footer()
+
+
+class HelpScreen(Screen):
+    def compose(self) -> ComposeResult:
+        yield Placeholder("Help Screen")
+        yield Footer()
+
+
+class ModesApp(App):
+    BINDINGS = [
+        ("d", "switch_mode('dashboard')", "Dashboard"),  # (1)!
+        ("s", "switch_mode('settings')", "Settings"),
+        ("h", "switch_mode('help')", "Help"),
+    ]
+    MODES = {
+        "dashboard": DashboardScreen,  # (2)!
+        "settings": SettingsScreen,
+        "help": HelpScreen,
+    }
+
+    def on_mount(self) -> None:
+        self.switch_mode("dashboard")  # (3)!
+
+
+if __name__ == "__main__":
+    app = ModesApp()
+    app.run()