Update our documentation.

[Alexhuszagh]

Add documentation for examples and additional enhancements.

• Document the titlebar, custom styles, and other examples.
• Document custom widget overrides like sliders, dials, and LCD numbers.
• Convert the Qt5 refs to Qt6.
• Add in better links to examples in the README.
• Add a changelog.
• Make our custom widgets, like titlebars and sliders, to use a general-purpose class and then reusable components.
• Ensure we use a shallow clone for our CMake builds.