New output format for the manual.

* Introduces a new HTML5 output format for the manual.
* Removes the PDF and text outputs.
* Removes the manual and Java APIdocs from the download packages.
* Gets rid of the native docbook dependency (but still requires xmllint).
* Removes the preview build.
Merge pull request #2167 from nawroth/new-manual

New output format for the manual.