Resolve topic fragments and code references (topic-fragment)

The topic-fragment step expands content references to elements in the same topic and resolves references made with the <coderef> element. This step is implemented in SAX pipes.

Content references to elements in the same topic are defined via same-topic fragments such as #./ID in URIs.

The <coderef> element is used to reference code stored externally in non-XML documents. During the pre-processing step, the referenced content is pulled into the containing <codeblock> element.