Docs: Update instructions for deploying using Apache with mod_wsgi

It might help using a configuration more similar to the ones:

* After installing Pootle using pip
* After installing Pootle using Fabric
Docs: Add WSGI script example for deployment using Apache with mod_wsgi

Fixes bug 2688
Docs: Change WSGI and Apache examples for providing a custom settings file

This allows having Pootle settings customizations in a custom location outside
Pootle code, perhaps along with the WSGI script.