[DashboardLayout] Add navigation actions

[apedroferreira]

Make it possible to render any content as an action in navigation items, such as badges or popover menus.

Also includes a few small unrelated general improvements to the documentation such as better example theme colors and initial page in the demos.

Closes #3608

Documentation: https://deploy-preview-3776--mui-toolpad-docs.netlify.app/toolpad/core/react-dashboard-layout/#navigation

Includes:

• Feature
• Documentation
• Test

[Janpot]

Follow-up PR:
The scrollbars in those preview windows are not a great UX, when I try to scroll past them, the code view takes over. We should avoid doing this. The easiest way will be to scope down the demos. Instead of creating 1 demo that shows badges + menus + buttons. Show a demo for each of them, with a non-scrolling preview. Essentially the same can be done for the first demo, instead of creating a showcase that displays all functionalities, create a subsection for each of them: page, folder, subheader, divider.

[Janpot]

Approving. We can improve the docs page in follow up.

[apedroferreira]

Approving. We can improve the docs page in follow up.

Got it, made an issue for improvements in #3836