docs: Updated custom UI example

[matthewlipski]

This PR reworks the custom UI example and adds a non-pro example for how to make a static formatting toolbar. The new custom UI example uses Material UI to implement replacements for the default Mantine components, and is extensively commented to show users how to customize elements and use other component libraries that aren't MUI.

Closes #867

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[YousefED]

Looks great!