[docs-infra] Support {{"component" markdown collocation

[oliviertassinari]

Summary

Current state: If you want to show a custom components in the docs with the design of the docs (not the default material design) you need to use {{"component": "modules/components/..." }}

We should be able to render custom components from the markdown in the same folder.

without having the default library theme, but the MUI branding theme.

As for the solution, one is to use {{"demo": but with an option to not reset the parent theme.

Examples

• [docs] Tweak Material UI's "Showcase" page design #37259 (comment)
• https://github.com/mui/mui-x/blob/266b74328e8184a85d02271d77208b792de20c6f/docs/data/whats-new/whats-new.md?plain=1#L5

Motivation

Make it easier to iterate on the logic, e.g. deleting all files at once.

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