[docs][joy] Improve the descriptions of props in API docs

[hbjORbj]

• I have followed (at least) the PR section of the contributing guide.

Preview: https://deploy-preview-36307--material-ui.netlify.app/joy-ui/api/alert/#slots

[mui-bot]

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• docs/data/joy/customization/themed-components/themed-components.md

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[siriwatknp]

the Props table

let's change the description of these props for all components

• color: (we need to create #extend-colors section in Themed components—Extend colors

  The color of the component. It supports those theme colors that make sense for this component.
  
  To learn how to add your own colors, check out [Themed components—Extend colors](https://mui.com/joy-ui/customization/themed-components/#extend-colors).

• size:

  The size of the component.
  
  To learn how to add custom sizes to the component, check out [Themed components—Extend sizes](https://mui.com/joy-ui/customization/themed-components/#extend-sizes).

• variant:

  The [global variant](https://mui.com/joy-ui/main-features/global-variants/) to use.
     
  To learn how to add your own variants, check out [Themed components—Extend variants](https://mui.com/joy-ui/customization/themed-components/#extend-variants).

Slots table

Needs #34990 to be merged. I think this can be a separate PR?

[hbjORbj]

Thanks, Jun.

• I completed the Props table based on your feedback.
• I added a extend-colors documentation: https://deploy-preview-36307--material-ui.netlify.app/joy-ui/customization/themed-components/#extend-colors
• I will add the changes for the Slots table in a separate PR.

[siriwatknp]

👍 Nicely done!