Add inline template documentation

[paltman]

EIther do this as one pass for break this up for each app, but the idea is we should design the pages in a way where you can view the template for styling but also be able to view the documentation for the template.

Template documentation should be pretty easy to generate as its very formulaic in that it's really just documenting the context variables, template name, and any blocks available to override.

Perhaps some form of tabbed interface for each template. The default is a preview but then a tab or other link were you can reveal the docs. Maybe the documentation can be represented well in a 300-600px column and if that's the case can provide a right-hand pop-out like the proposed left-hand pop-out menu in #15.

We would then need to codify the documentation as part of the configuration.

[paltman]

Things to document:

• context variables
• blocks available to override (typical pattern would be overriding, not extending the template)
• make the source of the template available for easy reference on structure for styling