Fix validation errors and make builds enforce it

This change updates the D&O guide docbook files to be schema-compliant,
and causes the build to fail on changes that introduce invalid output.
This also changes the link to the PDF within the HTML rendering to use a
relative path, so that builds from the CI system will link to the
proposed PDF content instead of the content stored on the production
branches.

Change-Id: I12c6400c0e80288d74aca304c4662c36a5ddb348