Add live-preview support for ref-guide files using asciidoc-includes

With the completion of SOLR-11032, the "Using SolrJ" page in the ref-guide now imports some Java code snippets from an external file. This is done to allow the snippets appearing in the documentation to be compiled/run as part of the build...so they will always stay up to date.

During the ref-guide build, this external file is copied into the ref-guide build directory so that the references to it can be resolved. However, anyone using the "live preview" feature of modern asciidoc editors (Atom, etc.), will find that the snippets don't appear in their live-preview. Depending on your editor, a discrete error message may be displayed.

It would be nice to fix this "papercut" in usability, or at least provide a workaround. The easier we can make it to improve the docs, the better.