All items have source links back to their actual code. Source files can be
omitted with the doc(html_no_source) attribute on the crate. Currently there is
no syntax highlighting, but that will come with syntax highlighting with all
other snippets.
Closes#2072
Removes old rustdoc, moves rustdoc_ng into its place instead (plus drops the _ng
suffix). Also shreds all reference to rustdoc_ng from the Makefile rules.