Improved .frow-container

Firstly, is `.frow-container` the best name? It got the name from back when Frow and Bootstrap were most identical, and was to just differentiate between Bootstrap's `.container' and Frow's. Now it's mostly used to make whole pages not go too wide. Can we perhaps rename this, and provide more options. Sometimes it's too wide or too skinny. Frow's own documentation website found it too wide, but other projects have found it too skinny. Perhaps some type of "-lg" "-sm" system?

The issue identifies the `.frow-container` class and Frow's documentation website as the starting points; review how the class is currently used and how its width affects the documentation site and other projects. Decide on the naming and size-option scope before implementation, with done defined as a documented container API that supports the required width variations without breaking existing usage.

Written by the indexing model from the issue text.