Replace references to the document field with references to the markdoc field in docs and templates

[emmatown]

Fixes #1162

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🦋 Changeset detected

Latest commit: 8a1e2f51f2b445baad3d511a1de9ee7b9b39e2d5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

| Name | Type | | --------------------------- | ----- | | @keystatic/templates-nextjs | Patch | | @keystatic/templates-astro | Patch | | @keystatic/templates-remix | Patch |

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