Cross reference link types

Added class attributes to cross reference links to support styling,
though did not make corresponding CSS changes. Class values include
tech-ref (technique), gl-ref (guidelines), understanding-ref
(understanding), ex-ref (working example), and test-ref (test file).
This closes ACTION-152 https://www.w3.org/WAI/GL/track/actions/152/.
File URIs for output doc

Previously removed file URIs from build.xml because it created certain
problems; needed to add them to the XSLT to get it to generate.
Latest version links

Adding latest version links to footer of sliced docs, and canonical
latest version link to head of all docs
Link to technology notes

Added link to technology notes, if it exists. It's provided along with a
link to user agent notes. However, it's subject to the current bug that
if user agent notes not present in the XML, the link not output; that
will hopefully be fixed separately. Alternatively, this link could be
added to the footer, but didn't do that for this commit. This closes
ACTION-188 https://www.w3.org/WAI/GL/track/actions/188.