SGF-305: Move to Asciidoctor for reference documentation

First cut at migrating reference docs to asciidoctor.
SGF-305 - Finished migration to Asciidoctor.

Upgraded Asciidoctor plugin to 1.5 and re-enabled Docbook plugin for PDF generation. Removed obsolete repository declarations. Use Spring Data Build shared resources to use the CSS file from it. Configure Docbook reference plugin to use Asciidoctor generated sources. Override expandPlaceholders to avoid replacement of variables in source files. Tweaked docs ZIP setup to include Asciidoctor generated HTML and Docbook generated PDF version.
Fixed up the build.gradle script based on the merged asciidoc PR.  Disabled the 'bundlor' Gradle plugin after attempting to get the 'bundle' task working, which led to several warnings due to an unspecified, non-customized 'Import' Bundle Manifest header.
Fixed up the Spring Data GemFire GitHub project page, correcting links, updating Spring Maven Repo URLs to repo.spring.io, switching references from the Spring Forum to StackOverflow and so on.