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

docs/widgets/list_item.md

@@ -1,4 +1,4 @@
-# List Item
+# ListItem
 
 `ListItem` is the type of the elements in a `ListView`.
 

docs/widgets/list_view.md

@@ -1,4 +1,4 @@
-# List View
+# ListView
 
 Displays a vertical list of `ListItem`s which can be highlighted and selected.
 Supports keyboard navigation.

docs/widgets/markdown_viewer.md

@@ -1,4 +1,4 @@
-# Markdown Viewer
+# MarkdownViewer
 
 A Widget to display Markdown content with an optional Table of Contents.