- 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

Moved DocNet code to its own folder
Added MarkdownDeep parser code plus tests
Migrated DocNet code to use MarkdownDeep. Not complete yet (the h2
headers aren't collected yet).