docs: fix updated at timestamps

[larsrickert]

relates to #1615

VitePress shows timestamps at the bottom of every page when it was last updated.

Previously when building the onyx docs, all pages had the same timestamp (build time) which was caused by a build error because of the createOnyx() Vue plugin which did not fully support server side rendering.

You can test this PR by running:

1. cd apps/docs
2. pnpm build
3. pnpm preview

Checklist

• The added / edited code has been documented with JSDoc
• A changeset is added with npx changeset add if your changes should be released as npm package (because they affect the library usage)

[changeset-bot]

🦋 Changeset detected

Latest commit: 2d49bdf

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

This PR includes changesets to release 7 packages

Name	Type
sit-onyx	Major
@sit-onyx/nuxt	Major
demo-app	Patch
playground	Patch
@sit-onyx/chartjs-plugin	Major
@sit-onyx/storybook-utils	Major
@sit-onyx/vitepress-theme	Major

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