[Bug] NamespaceLayout behavior change

[CypherPotato]

Describe the bug
Apparently the compiler no longer respects the namespaceLayout property after version 2.78.1. After upgrading from version 2.76.0 -> 2.78.1, the API TOC generation changed its layout. No changes were made to docfx.json.

Expected behavior
It was expected to remain the same as before.

Old layout (0.76.0):

New layout (0.78.1):

Context (please complete the following information):

• OS: Windows
• Docfx version: 0.78.1

Used docfx.json: https://github.com/sisk-http/docs/blob/master/docfx.json

[filzrev]

It seems TOC structure for mref output format is changed by #10090.

This PR add type attribute to TOC view model.
And docfx template grouping toc nodes based on type categories. (Insert Namespaces/ Classes node)
(It's transformed only when "memberLayout": "separatePages" option is used.)

Currently there is no option to control these behaviors.
It might be better to add options to change behavior.