[docs-infra] Change CSS vars generation to be extracted from Enum

[mnajdova]

This PR updates the CSS vars docs generation, to depend on an enum, as it offers better representation in comparison with interface + const. The enum could have the following form:

enum AccordionCssVars {
  /**
   * The max width of the Accordion.
   * @type {number}
   */
  maxWidth = '--max-width',
  foo = '--foo',
}

As we may not want to expose this enums as a public API, the script is updated to look for specific files in order to find the enum. The usage can be find in mui/base-ui#874.

[mui-bot]

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[mnajdova]

@alexfauquette we decided to change a bit the processing of the CSSVars type:

• use enum, it better fits the purpose, no need for a const
• we didn't want to include this in the public API, so we are looking into a specific file name convention (CssVars|Classes) ending

I added example to showcase the change.