Build docs in github workflows

[lkostrowski]

Currently, build is performed in Vercel which is limited to org members.

Not every docusaurus error is thrown locally, for some reason anchors check can pass locally and fail on Vercel

This is also worse for external contributors.

To improve that, we can:

1. Build project in Github Action → it will give us early feedback visible within PR
2. Optionally push built artifact to vercel, to avoid builing 2nd time (https://vercel.com/docs/cli/deploy)