Set up site preview on PRs, e.g. with Netlify

[rgommers]

We had Surge but it never really worked. What we want is an integration that deploys each PR on Netlify, so we can browse the site and inspect changes before deploying it to numpy.org

When doing this (or before), the Surge link can be removed from .github/pull_request_template.md

[rgommers]

Netlify preview added in gh-347

[rgommers]

Cleaning up the Surge left-overs in gh-348. The main work is done here, so closing.

[rgommers]

Given that this is the canonical issue for Netlify previews, adding the link to today's digging about how/where has admin access to this: #887 (comment)