[docs] Document Base UI use cases

Describe scenarios in which Base UI can be used and recommend a customization solution for each of them.

Examples include:

1. Styling a single project without the need for further customizability.
2. Creating a design system to be used across several projects. Limit the customization options to maintain consistency and prevent developers from inventing their own design choices.
3. Creating a design system template (akin to Joy UI), to build different design systems on. Maximize customizability.