Hide some members from the public docs. (#3080)

* Hide some members from the public docs.

See relevant issue: #3076.
Some methods need to be implemented to make the widget work but the user doesn't really care about them. For that matter, we can hide them from the public documentation.

* Use private handler to hide from docs.

Related comments: https://github.com/Textualize/textual/pull/3080#issuecomment-1671129733

docs/api/widget.md

@@ -1 +1,5 @@
 ::: textual.widget
+    options:
+        filters:
+          - "!^_"
+          - "^__init__$"

mkdocs-common.yml

@@ -78,6 +78,15 @@ plugins:
             - "!^_"
             - "^__init__$"
             - "!^can_replace$"
+            # Hide some methods that Widget subclasses implement but that we don't want
+            # to be shown in the docs.
+            # This is then overridden in widget.md so that it shows in the base class.
+            - "!^compose$"
+            - "!^render$"
+            - "!^render_line$"
+            - "!^render_lines$"
+            - "!^get_content_width$"
+            - "!^get_content_height$"
     watch:
       - mkdocs-common.yml
       - mkdocs-nav.yml

src/textual/widgets/_placeholder.py

@@ -120,7 +120,7 @@ class Placeholder(Widget):
         while next(self._variants_cycle) != self.variant:
             pass
 
-    def on_mount(self) -> None:
+    def _on_mount(self) -> None:
         """Set the color for this placeholder."""
         colors = Placeholder._COLORS.setdefault(
             self.app, cycle(_PLACEHOLDER_BACKGROUND_COLORS)