Add RelativeSourceFileName and RelativeTargetFileName variables for the htm template

Fix broken headings in Markdown files

- 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.

See it in action here:
http://www.llblgen.com/documentation/5.0/Designer/index.htm

Merge of PR #31

9473a8fee4

1affa872af