Set up PR previews

[hellcp]

This is useful for verifying the changes from the PR build and look good in the reviewers browser. There are a couple of options:

1. https://docs.netlify.com/deploy/deploy-types/deploy-previews/
2. https://vercel.com/docs/git/vercel-for-github#preview-urls-for-the-latest-changes-for-each-pull-request
3. https://developers.cloudflare.com/pages/configuration/preview-deployments/
4. https://render.com/docs/service-previews#working-with-pr-previews
5. https://github.com/marketplace/actions/deploy-pr-preview (probably a bad idea until v2 comes out)

[Sorixelle]

The issue with a lot of these types of tools IME is that they want to take over the entire build/deploy process. We've got a pretty advanced build pipeline that involves generating SDK docs at the moment, so we really need to be able to hook into Github Actions to do our deploys. I think it's possible with all of these tools - we should figure out which ones make this easiest.

I'm also going to preemptively rule out Vercel and Cloudflare - I would rather not do business with those companies if we can avoid it.

[Sorixelle]

Was doing some testing on a fork and I just realized another problem - if we want to use Github Actions to deploy PRs, then we have to build and deploy pull requests on pull_request_target, otherwise the workflow won't have our deploy token and it won't be able to work. That's a recipe for secret exfiltration though someone pushing code with a malicious Jekyll plugin or something, so I don't really want to do that. I'm not sure we can look at this properly until we simplify the build process, and move SDK docs generation out of this repo.