Move docs.sourcegraph.com templates/assets into sourcegraph repository

Created by: sqs

Previously these lived in https://github.com/sourcegraph/docs.sourcegraph.com and were updated when you pushed to that repository's branch named docs.sourcegraph.com. Now they live in this repository (to simplify the release process) and are served from this repository's master branch (to avoid confusion where people forgot to push to the specially named branch).

Fix #2465

Before merging this, I will copy the latest docs.sourcegraph.com repository contents over here (in case that repository is changed in the intervening time).