Add Customizing Documentation

[scurker]

We should document how we expect authors to customize or override Cauldron styles. Some examples:

• https://mui.com/material-ui/customization/how-to-customize/
• https://carbondesignsystem.com/elements/themes/overview/
• https://www.lightningdesignsystem.com/platforms/lightning/styling-hooks/

It's become a somewhat common practice to override styles via CSS specificity:

// in application code
.Component {
  background-color: red;
}

This can come with its own set of issues if not used correctly as it would be expected that authors would correctly update any styles that need to cascade. There are natural ways to do this with css vars, but we should document each approach and specify how and when an author should override or customize Cauldron styles.