Command Palette tweaks and docs (#3289)

* renames to command palette and docs

* docs

* simplifyt

* note

* docstring

* Update src/textual/command.py

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

* Update docs/examples/guide/command_palette/command01.py

Co-authored-by: Dave Pearson <davep@davep.org>

* populate text

* screen commands

* Update docs/guide/command_palette.md

Co-authored-by: Dave Pearson <davep@davep.org>

* Update docs/guide/command_palette.md

Co-authored-by: Dave Pearson <davep@davep.org>

---------

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

docs/examples/guide/command_palette/command01.py

@@ -0,0 +1,65 @@
+from pathlib import Path
+
+from rich.syntax import Syntax
+
+from textual.app import App, ComposeResult
+from textual.command import Hit, Hits, Source
+from textual.containers import VerticalScroll
+from textual.widgets import Static
+
+
+class PythonFileSource(Source):
+    """A command source to open a Python file in the current working directory."""
+
+    def read_files(self) -> list[Path]:
+        """Get a list of Python files in the current working directory."""
+        return list(Path("./").glob("*.py"))
+
+    async def post_init(self) -> None:  # (1)!
+        """Called once when the command palette is opened, prior to searching."""
+        worker = self.app.run_worker(self.read_files, thread=True)
+        self.python_paths = await worker.wait()
+
+    async def search(self, query: str) -> Hits:  # (2)!
+        """Search for Python files."""
+        matcher = self.matcher(query)  # (3)!
+
+        app = self.app
+        assert isinstance(app, ViewerApp)
+
+        for path in self.python_paths:
+            command = f"open {str(path)}"
+            score = matcher.match(command)  # (4)!
+            if score > 0:
+                yield Hit(
+                    score,
+                    matcher.highlight(command),  # (5)!
+                    lambda: app.open_file(path),
+                    help="Open this file in the viewer",
+                )
+
+
+class ViewerApp(App):
+    """Demonstrate a command source."""
+
+    COMMAND_SOURCES = App.COMMAND_SOURCES | {PythonFileSource}  # (6)!
+
+    def compose(self) -> ComposeResult:
+        with VerticalScroll():
+            yield Static(id="code", expand=True)
+
+    def open_file(self, path: Path) -> None:
+        """Open and display a file with syntax highlighting."""
+        syntax = Syntax.from_path(
+            str(path),
+            line_numbers=True,
+            word_wrap=False,
+            indent_guides=True,
+            theme="github-dark",
+        )
+        self.query_one("#code", Static).update(syntax)
+
+
+if __name__ == "__main__":
+    app = ViewerApp()
+    app.run()