This PR adds a `type = "posts"` for themes that need it.
Most notably [Hyde Hyde](https://themes.gohugo.io/theme/hyde-hyde/) that currently has its index page empty will have it populated with posts once again.
If more themes need different content types I will see whether it is possible to turn `type` into an array to add more of these.
This reverts commit 4e47e94a69.
A [great number](https://app.netlify.com/sites/hugothemes/deploys/5c6936d1dddaef000819a6e8) of theme demos on the website broke because of this commit due to the following:
- Series is on the predefined frontmatter variable lists due to its use in the internal opengraph template.
- Series is predefined as an array whereas I entered it as a string.
- Also series needs to be defined as a Taxonomy in the config of an exampleSite. Since the config is inherited by theme authors if they have not defined the series taxonomy then their demo will again break.
**NOTE**
I will be more rigorous with testing before sending more PRs.
cc: @digitalcraftsman
