Instruct reader to add VM templates so parsing is successful

The problem was coming from code we were inserting that was pointing to non-existent templates. I added instructions to add blank `header_detailed.vm` and `header_brief.vm` template files so parsing would complete successfully.

I also added additional text to make sure reader is modifying `portal_normal.vm` in the `_diffs` directory.

https://issues.liferay.com/browse/LRDOCS-794