Add support to revalidate single page using builtin webhook

[kierzniak]

Hi,
I would love to see feature where I'm able to refresh single DSG page using builtin webhook. Gatsby is able to refresh content using __refresh webhook for development purpose but not for production build.

Let's say I have

• Gatsby website example.com
• WordPress CMS cms.example.com

I'm hosting two apps by my own. WordPress using standard PHP hosting and Gatsby using nodejs server and gatsby serve command.

When I update WordPress CMS instead of triggering build I would like to send webook to Gatsby website e.g. example.com/api/refresh?page=about-us to refresh GraphQL data and remove single DSG page. Next visit to this page will generate new page with updated content using fresh GraphQL data.

Something similar has Next.js: https://nextjs.org/docs/basic-features/data-fetching/incremental-static-regeneration#using-on-demand-revalidation

I believe this feature can dramatically increase performance and user experience. I think revalidating single page server side will be faster than creating new build, even incremental. Also CMS user might see the result immediately not after build so user experience will be better.