feat(v2): allow to define custom CSS class for Tabs component

[Simek]

Motivation

Currently Tabs are manually added by the users to the docs pages, but the customization (without introducing additional wrapper) is limited to the "block" and "non-block" appearance.

This simple PR adds the ability to pass className prop to Tabs, which if present, will be added at the end of class chain providing the users simple, unique CSS hook to customize the styling of selected Tabs block.

I have also added a small section about this feature to the docs on the Markdown Features page. It includes a brief description and an example usage (which required to add CSS to the custom.css file), here's the preview:

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Docusuaurs V2 website running locally with the added in example on the Markdown Feature page.

Related PRs

N/A

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[lex111]

LGTM

[slorber]

Thanks.

Wonder if it's worth to write as much doc for just passing a className prop.
I think we should forward className / style by default for all React elements that are advised to use in MDX.
But maybe this doc will be helpful for non-React users as we have a diverse user base so let's merge this.