lib: Let NodeInterface extend RecursiveIterator

Since a node may have children and they should be iterable, the Nodefinterface
now extends RecursiveIterator.
lib: Remove TreeIterator

Nodes implementing the NodeInterface (which extends the RecursiveIterator interface)
are already iterable.
lib: Add Data/Tree/Node.php
lib: Add a not yet customizable node renderer
doc: Remove class DocToc as it's superseded by Data/Tree/Node

refs #4820
doc: Remove DocTocHtmlRenderer as it's superseded by Data/Tree/NodeRenderer

refs #4820
doc: Don't use absolute 'use' in MarkdownFileIterator

refs #4820
doc: Implement DocParser::getChapter()

refs #4820
doc: Fix actions and view scripts DocParser usage

refs #4820