Add Gatsby docs config (sidebar link, etc.) for full-query caching from

#2437.

While #2437 did introduce the documentation and the appropriate
configuration for it, it did so while we were still utilizing Hexo for our
docs, and the configuration changes necessary for our docs' Gatsby-ification
didn't happen.

This puts the configuration in place for Gatsby and merges into the 2.5.0
release branch.

Ref: https://github.com/apollographql/apollo-server/pull/2437/files#diff-b09bbff3e688a10b35f7de810d65c28e

docs/gatsby-config.js

@@ -28,6 +28,7 @@ module.exports = {
             'essentials/data',
           ],
           Features: [
+            'features/caching',
             'features/mocking',
             'features/errors',
             'features/data-sources',