Add CSSBaseline to the default layout for better UI

[fzaninotto]

Problem

MUI's <CSSBaseline> is crucial to get a good UI with react-admin, yet we never mention it in the documentation.

We do our tests with the Simple example, which uses CSSBaselaine, without realizing that for most users, without CSS Baseline, the UI is degraded.

Solution

Enable it by default

Before

Note the top margin between the AppBar and the content

After