Rename template.html => pandoc_template.html

We have several templating systems: Jekyll's on-the-fly templating based
on the templates under `_layouts/`; pandoc for generating html from the
markdown sources that we haven't ported to Jekyll yet; and soon we'll be
adding a step to import markdown docs from the stb-tester git repo.

This commit makes it a bit easier to follow which template is for what.