New persistence guides

We’ve added a few more Scalatra Guides dealing with persistence over the past few weeks.

There’s one on MongoDb, and one on Riak.

Also, we’ve written a guide which goes over the general case. There are probably a few dozen libraries which could be used for persistence with Scalatra, and the number of specific guides we currently have (4) doesn’t really reflect the richness of options or show how easy it is to get started with whichever one you choose - so the introduction to persistence guide gives an overview of the steps for writing your own.

If anybody would like to contribute a guide for a specific persistence library which we don’t have docs for, pull requests on the docs are very welcome.