Documentation - styling editor should be updated to Mui V5 format

[chrishj59]

Checklist

• [ x] This is not covered in the repository examples.
• [ x] I am using the package latest version.
• [ x] This is a bug report or an enhancement proposal.

Description

Write here your issue description.
The documentation on the structure of the theming guide Styling the editor follows the Mui V4 format and should follow V5

Change from v4:
overrides: { MUIRichTextEditor: { root: { marginTop: 20, width: "80%" }, editor: { borderBottom: "1px solid gray" } } }

Add Mui V5 structure:
components: { MUIRichTextEditor: { styleOverrides: { root: { marginTop: 20, width: "80%" }, editor: { borderBottom: "1px solid gray" } } } }