docs_app: Switch docs app to Jinja2 templates

They're a bit more powerful.

Specifically, we can use a loader to make specifying template paths cleaner.
Also, we can choose to escape or not escape markup with the |e tag filter.

This allows us to embed the simple push example in both code and live form.
Merge pull request #11 from AdrenalineBrowser/docsapp_use_jinja2

docs_app: Switch docs app to Jinja2 templates