[docs] Use same infra from v5

[DanailH]

Based on this commit 50a16a1

Preview: https://deploy-preview-2441--material-ui-x.netlify.app/components/data-grid/

[DanailH]

I was trying to make the docs work, compile. So this is that effort

[m4theushw]

Finally the docs is using the same infra from the core v5. There're still some small things that I'll fix in a follow-up. This PR is already giant.

• Trying to edit the Filled Quantity in https://deploy-preview-2441--material-ui-x.netlify.app/components/data-grid/demo/ crashes because it can't find the element to focus
• Since the size of the IconButton increased in v5, the title of the columns on some demos is cutted: [DataGrid] Drop v4 support #2515 (comment)
• The color of the links in https://deploy-preview-2441--material-ui-x.netlify.app/components/data-grid/events/#catalog-of-events is wrong because the docs uses a custom theme but the default one on demos
• Although it runs the demos with v5, opening them in CodeSandBox will still use the old @material-ui/core

[oliviertassinari]

Looks like a great first iteration. This one required to dive deep into the structure of the repositories, well done

[oliviertassinari]

Also, it's awesome that we finally reconcile the docs of MUI X with MUI Core 🎉. A couple of feedback to the core:

1. I think that we have the same side nav issue as with the date picker. It's not very clear what "Demo" refers to. cc @danilo-leal I didn't follow what was the conclusion to it:

https://deploy-preview-2441--material-ui-x.netlify.app/components/data-grid/demo/

2. Regarding the not correctly themed links in https://deploy-preview-2441--material-ui-x.netlify.app/components/data-grid/events/#catalog-of-events. Maybe we need to expose a branding theme provider component to wrap this demo cc @mnajdova, @siriwatknp.

[danilo-leal]

1. I think that we have the same side nav issue as with the date picker. It's not very clear what "Demo" refers to. cc @danilo-leal I didn't follow what was the conclusion to it:

Let me quickly separate the answer into two topics:

1. Visual: in regards to the visual conclusion of nested levels, we haven't yet implemented it yet but I left a simple suggestion on the previous Markdown polish PR for addressing it.
2. Content: we decided to create a new PR and not use the Markdown polish PR for rearranging the grouping of links on the side nav. We also haven't done it yet. But reflecting specifically about the Data Grid Demo page, I'm not sure if we need to have it stand-alone, it seems weird. I'd think that it makes more sense to have a Demo section probably on the Getting Started page or something like that. I went to GA looking for pageviews and, in the last month, Demo had 18k and Getting Started had 16k. They're not too far away so I suppose it wouldn't be that much of a problem to merge them?!

Anyway, I think we should probably create a second Docs polishments PR for addressing these two (visual solution for nested levels on the sidebar and content regrouping) and other small details left open from the first PR. We can move these discussions further there. It would be nice to do that before v5 stable. How do you feel about it? @mnajdova @siriwatknp

[flaviendelangle]

Why did we rename the "Overview" page "Overview" ? It's creating visual redundancy.