Consider root and current section's content type if set in front matter

Consider root and current section's content type if set in front matter

This should allow for less duplication of templates. Before this commit it was possible to override the content page of a given page/section, but only one page at a time.

Full "template sets" can now be inherited by setting `type: blog` etc. in the section content page's front matter, and that type will be considered when looking for layouts for all pages in that section.

For nested sections, it will use consider both `type` set in the current section first, then `type` set in the first section below home, e.g. `/docs`.

This commit also adds a new Page method: `FirstSection`. This navigates up to the first section below home (e.g. `/docs`). For the home page it will return itself.

Fixes #4891