Fix ZEIT/now to Vercel

[seisman]

Description of the problem

We use the ZEIT/now service for continuously building the documentation for each commit. Recently, ZEIT changed their name to Vercel (https://vercel.com/blog/zeit-is-now-vercel). The configuration file now.json also changed to vercel.json. We may need to update it accordingly.

I may be wrong here, as I didn't check the documentation carefully.

[weiji14]

Yes, I did notice it a while back, but everything still seems to work using the Github Now App. There doesn't seem to be a Vercel app (yet?), but there is a Github Action at https://github.com/amondnet/vercel-action that does the same thing I guess.

So we probably don't need to change anything for now. But it would be nice to move to the Github Action at some point as it should allow us to trigger the builds via a command (like /now build?) instead of every commit.

[seisman]

It seems the only thing we need to do is renaming "now.json" to "vercel.json" (although now.json is still supported).

As for the vercel-action, I don't see any benefits to switch to the action. As per the documentation (https://github.com/amondnet/vercel-action#disable-vercel-for-github), we still need the "vercel.json" file to disable the vercel-github integration. Using the vercel-action means we need three files "vercel.json", "package.json" and the workflow file.