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
Site.assembleSections logic assumes that the the home page would always be the first in the Site's list of pages. This is not in fact guaranteed to be true. When it is not, the method can fail to set the parent for some or all root-level pages.
Fixes #4447
This commit fixes an issue introduced in the recently released Hugo 0.22.
This logic did not handle the case with root sections with non-section subfolders very well.
Fixes #3586
