CurrentSection should navigate sideways for all branch nodes (also kind=taxonomy)

[bep]

This issue is another part in an ongoing cleanup/consolidating job.

Given the below content tree, assuming that categories is defined as a taxonomy):

content
├── categories
│   └── _index.md
└── posts
    ├── _index.md
    └── p1.md

Current behaviour:

• (site.GetPage "posts").CurrentSection == (site.GetPage "posts")
• (site.GetPage "posts/p1").CurrentSection == (site.GetPage "posts")
• (site.GetPage "categories").CurrentSection == site.Home

So, categories isn't defined as a section, but having it behave differently in this context, is confusing.

This also means that calling .FirstSection on a term page will return the root taxonomy node (and not the home page).