[docs]: add scrollOffset in site config documentation

[Barbapapazes]

Describe the bug

Hello 👋,

Not really a bug but an API missing in the documentation. I'm building a custom theme and it took me an hour to understand why a scroll-margin-top does not affect the scroll. It seems the router overwrite it with a custom value offset, defined for the documentation theme (134px). For a custom theme this value will change but I found nothing in the documentation.

See the link below that shows all references to scrollOffset. The option exists but not in the documentation.

Reproduction

https://github.com/search?q=repo%3Avuejs%2Fvitepress%20scrollOffset&type=code

Expected behavior

I expect the documentation to have this API explained.

System Info

https://vitepress.dev/reference/site-config#site-config

Additional context

No response

Validations

• Check if you're on the latest VitePress version.
• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.