Bug 1478711 - Vagrant: Open port 8001 for sphinx-autobuild

Since the port was changed in #3813 from 8000 to 8001 (presumably so
it didn't clash with Virtualbox when run directly from the guest),
however this broke running the docs from inside the VM (for those
who don't want to install the docs dependencies on the host).

Vagrantfile

@@ -7,13 +7,15 @@
 Vagrant.require_version ">= 1.9.0"
 
 Vagrant.configure("2") do |config|
-  # for webpack-devserver access from host
+  # webpack-dev-server
   config.vm.network "forwarded_port", guest: 5000, host: 5000, host_ip: "127.0.0.1"
-  # for web server access from host
+  # Django runserver/gunicorn
   config.vm.network "forwarded_port", guest: 8000, host: 8000, host_ip: "127.0.0.1"
-  # for DB access from host
+  # Docs dev server
+  config.vm.network "forwarded_port", guest: 8001, host: 8001, host_ip: "127.0.0.1"
+  # MySQL
   config.vm.network "forwarded_port", guest: 3306, host: 3308, host_ip: "127.0.0.1"
-  # for Elasticsearch access from host
+  # Elasticsearch
   config.vm.network "forwarded_port", guest: 9200, host: 9201, host_ip: "127.0.0.1"
 
   if !Vagrant::Util::Platform.windows?