[docs-infra] Support "synced blocks"

[oliviertassinari]

Summary

Make is possible to share the same content between different pages of the docs without having to close them.

Examples

• GitHub: https://github.com/github/docs/blob/main/content/desktop/installing-and-authenticating-to-github-desktop/authenticating-to-github-in-github-desktop.md?plain=1#L18. They allow parameterizing the synced blocks too, to be able to personalize them based on where they will be rendered.
• Notion synced blocks https://www.notion.so/help/synced-blocks
• Stripe: https://markdoc.dev/docs/partials

Motivation

Avoid out of sync content, duplication of effort.

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