Add htmlbook and link the asciidoctor-backends repo to template examp…

…les (PR #809)

docs/_includes/templates-api.adoc

@@ -22,4 +22,4 @@ To use Haml as the template engine, you'd name the templates `document.html.haml
 Templates for block elements, like a paragraph or sidebar, are named after the block they handle.
 For instance, to override the default paragraph template with an ERB template, put a file named `paragraph.html.erb` in the template directory you pass to the `Document` constructor using the `:template_dir` option.
 
-For more usage examples, see the (massive) {tests}[test suite].
+For more usage examples, see the {backend-git}[asciidoctor-backends], the O'Reilly {uri-asciidoctor-htmlbook}[htmlbook] project and the (massive) {tests}[test suite].

docs/user-manual.adoc

@@ -114,6 +114,7 @@ Sarah White <https://github.com/graphitefriction[@graphitefriction]>; Dan Allen
 :uri-asciidoctor-pdf: https://github.com/asciidoctor/asciidoctor-pdf
 :uri-asciidoctor-pdf-readme: https://github.com/asciidoctor/asciidoctor-pdf/blob/master/README.adoc
 :uri-asciidoctor-pdf-theming-guide: http://gist.asciidoctor.org/?github-asciidoctor%2Fasciidoctor-pdf%2F%2Fdocs%2Ftheming-guide.adoc
+:uri-asciidoctor-htmlbook: https://github.com/oreillymedia/asciidoctor-htmlbook
 
 [NOTE]
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