Use smart variables in the documentation

[rmglennon]

This is something that is very common in large content management systems, but sigh...

We need a way to insert variables into documentation and easily update the text in one place and have it propagate.

This kind of thing is commonly used for a product name or something that might change over time, or during the development cycle. Longer text strings are often known as conrefs.

Right now, I need it for a base URL for the search demo site. I ended up doing a find and replace throughout the whole project. See https://github.com/pelias/pelias-doc/pull/195