Introduce a netlify.toml configuration to fix docs generation on Netlify.

Without this file, Netlify was choosing to build the documentation using the
npm scripts present in the root `package.json`, rather than the `package.json`
in the `docs/` directory, as it should be.

While it's strange that this behavior just cropped up around a recent swap to
using `@apollographql/apollo-upload-server`, I suspect that the problem is
more deeply rooted than that and we were just accidentally getting away with
the past behavior - which was defined less explicitly in the Netlify UI.

Now, with this `netlify.toml` file, we can run the exact build step we need
for documentation and also gain the ability to have per-branch control over
that configuration, which has been necessary in the past.

netlify.toml

@@ -0,0 +1,6 @@
+[build]
+  base    = "docs/"
+  publish = "docs/public/"
+  command = "npm install && npm run build"
+[build.environment]
+  NPM_VERSION = "6"