docs: Use theme from npm, rather than submodule, using chexo.

The theme which the docs in this repository are based on
([`hexo-theme-meteor`](https://github.com/meteor/hexo-theme-meteor)) has been
published to npm.

Additionally, most of the configuration which was once present in this
repository's `_config.yml` file, has been moved into an Apollo-centric theme
configuration npm,
[`apollo-hexo-config`](https://www.npmjs.com/package/apollo-hexo-config) (and
similarly, [GitHub](https://github.com/apollographql/apollo-hexo-config)).

The theme bundled in this npm is a converged version of the two `apollo`
and `master` branches on the `hexo-theme-meteor` repository, which is a result
of the work undertaken in meteor/meteor-theme-hexo#51.

Lastly, this makes use of a new `chexo` npm which acts as a wrapper for `hexo`
but permits the use of an inherited configuration from npm, rather than a
statically defined CLI file parameter (previously with `--config a,b`).

docs/package.json

@@ -6,6 +6,8 @@
     "version": "3.5.0"
   },
   "devDependencies": {
+    "apollo-hexo-config": "^1.0.2",
+    "chexo": "^1.0.4",
     "handlebars": "^4.0.5",
     "hexo": "3.5.0",
     "hexo-generator-archive": "0.1.5",
@@ -17,12 +19,13 @@
     "hexo-renderer-marked": "0.3.2",
     "hexo-server": "0.3.1",
     "lodash": "^4.13.1",
+    "meteor-theme-hexo": "^1.0.1",
     "nodemon": "^1.14.12",
     "showdown": "^1.4.2"
   },
   "scripts": {
-    "start": "hexo serve",
-    "build": "hexo generate",
+    "start": "npm run build && chexo apollo-hexo-config -- server",
+    "build": "chexo apollo-hexo-config -- generate",
     "develop-theme": "nodemon -x 'rm db.json; hexo serve' -w assets/ -w code/ -w source/ -w themes/ -w scripts/",
     "clean": "hexo clean",
     "test": "npm run clean; npm run build"