[#520] First swing at the theming docs
[#520] Minor textual tweaks
[#520] Jinja2 not Jinja
[#520] Removes doc/extenstion-templating.html as it's inaccurate and wrong
[#520] Few more textual tweaks
[#520] Added more context to how to add templates within an extension
[#520] Added blank line for better code rendering
Merge branch '520-templating-docs'