This setting makes the name of the "Home" page configurable.
Example usage in docnet.json:

    "StartingPageName": "Getting Started",

This was confusing and unnecessary and created a useless space in root-referenced pages with 2+ levels of headings.

So currentrelativeroot has been removed, all elements which had that css class now have currentrelative as css class.

# Conflicts:
#	src/DocNet/INavigationElement.cs
#	src/DocNet/NavigatedPath.cs
#	src/DocNet/NavigationElement.cs
#	src/DocNet/NavigationLevel.cs
#	src/DocNet/SimpleNavigationElement.cs

- v0.14 version bump (not done yet)
- clean up of projbook project files on disk
- fix in path read in @snippet statement
- added references to System.Collections.Immutable to several projects
as it's indirectly loaded and will make docnet fail at runtime if
missing

Implements #38

Implements the @snippet statement, with tests.

Fixes issue: Nesting groups without index pages could lead to
'Collection was modified' exception.

Adds permalink anchor inside all header HTML tags. Themes are adjusted
to have a 'headerlink' css class to deal with this anchor: it's visible
only when hovered.