Add view source button to codeblocks

Many of the new docs (e.g. SDK 3.0) take advantage of Antora's
`include::3.0@python-sdk:example$blah.py`
feature to get several code snippets from the same, single code file for several separate code snippets.

In many cases, particularly where code makes up a complete, worked example, customers will want to grab the complete code listing.
Rather than have to write a manual link to the code file to appear at the end of each snippet, it would be useful to have Antora automatically generate some form of 'View the complete sample source' button or link, in response to a simple flag in the [source] declaration (or elsewhere).
... linking to the complete source file in the GitHub examples folder.

(in addition to the need for a 'copy to clipboard' for the snippet itself)