[Docs] Add color sample to Default Theme docs page

[mlizchap]

I think it would be helpful to have a sample of the default theme colors in it's docs page.

• I have searched the issues of this repository and believe that this is not a duplicate.

Summary 💡

Examples 🌈

Currently the docs just have the hex code with no preview of the color:

I think it would be helpful to have a preview like they do in code editors such as VS code:

Motivation 🔦

I reference the default theme a good bit and frequently have to manually search the colors. I think this would be helpful and save time.

I'm up for working on this if the maintainers think it's a good idea.

[eps1lon]

I had a similar idea today but regarding IntelliSense in vscode 😆

But this is a great idea now that we control the full component. I guess we can check if the string would be useful as a color and then add a preview:

https://github.com/mui-org/material-ui/blob/5633c2768437604c7cfeed4eb62493c537469b19/docs/src/pages/customization/default-theme/DefaultTheme.js#L45-L46