More documentation tweaks (#2001)

* Change the title of some widget references to their class names

The widget reference index had a slight mixture of widget title styles, some
being their class name, others being "written English" type titles. This
settles on the titles always being the class name.

* Add a link to the ContentSwitcher to the widget gallery

No screenshot for this, on purpose. By definition this widget isn't visible,
except via other widgets that it's enclosing and handling.

* Have mkdocs watch the main nav file

* Mark the content switcher off the roadmap

mkdocs-common.yml

@@ -80,6 +80,7 @@ plugins:
             - "!^can_replace$"
     watch:
       - mkdocs-common.yml
+      - mkdocs-nav.yml
       - mkdocs-offline.yml
       - mkdocs-online.yml
       - src/textual