[docs] Allow for the source code to be visible

[DanailH]

Feedback received: It would be nice to see the source code from https://mui.com/x/react-data-grid/demos/real-time-data/#

This is feedback that was provided through the feedback form located at the bottom of the page. This allies to the other example under Demos as well

[prakhargupta1]

I noticed on Slack that multiple users have requested for the code. While we figure out the multi-file solution, should we just link the source folder: https://github.com/mui/mui-x/tree/master/docs/src/modules/components/demos/data-grid/StockDashboard below this demo?

Although users can find it themselves, but some clearly aren't.

[alelthomas]

I'm reopening this PR https://github.com/mui/mui-x/pull/19759/ to merge as a first version, I think it's good. As an add-on, we can just add the GitHub icon next to the demos titles for easy access to the source code

[github-actions[bot]]

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