* 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
As pointed out in #1649, a `ChildrenUpdated` message is documented but it
doesn't exist in the code. It looks like it got added during development,
then removed after it was realised it wasn't needed, but presumably it got
left in the docs:
https://github.com/Textualize/textual/pull/1143#discussion_r1030409026
