[docs] Add docs-infra Docs

[dav-is]

Split off from #379 (only code) + #684

Read the docs: https://github.com/mui/mui-public/blob/davis/add-docs-infra-docs/docs/app/docs-infra/page.mdx

To see the demos:

• Install deps: pnpm i
• Build docs-infra: pnpm -F './packages/docs-infra' run build
• Start the dev server: pnpm run docs:dev
• Go to http://localhost:3000

[oliviertassinari]

For where to deploy this to, I would propose we have it under https://tools-public.mui.com/docs-infra/:

• We will need to support subfolder deployment to have components split in different repositories
• There doesn't seem to be a need to have another deployment instance