[docs-infra] Move BrandingCssVarsProvider to _app.js

[Janpot]

Make branding theme provider reusable without:

• unnecessarily recreating theme objects on each usage
• rendering components that are meant to be singletons (<NextNProgressBar />, <CssBaseline />, <SkipLink />...)

The goal is to be able to reuse as much as possible from the branding theme when we style things like the cookiebanner which should appear on every page, but live outside of the main branding theme provider

[mui-bot]

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Bundle size report

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

Generated by 🚫 dangerJS against 1e0891c

[oliviertassinari]

Move BrandingCssVarsProvider to _app.js

As far as I know, we can't do this change because we need docs/pages/_app.js to be barebone so when we create paygrounds we see default behaviors, e.g. theme of our components.

There is probably a root cause that we need to fix for our users too anyway?

[Janpot]

yeah, change of tac. Exporting a BrandingCssThemeProvider and make it efficient by building the theme only once (or at most twice if rtl option is set)

[dav-is]

This change was merged with #47744