lib: Add NodeInterface
lib: Add TreeIterator
doc/lib: Add DocToc class representing a toc

refs #4820
doc/lib: Add DocTocHtmlRenderer class to render a toc to HTML

refs #4820
doc: No longer use a partial for viewing a toc

refs #4820
doc/parser: Return the DocTocHtmlRenderer instead of an array

refs #4820
doc: Add view script for the toc of Icinga Web 2's doc

refs #4820
doc: Add a view script for module's doc toc

refs #4820