AsciiDoc multi-page splitter: generate links to directories instead of links to 'index.html'

The AsciiDoc multi-page splitter splits a single generated HTML page into multiple pages, one per source file. It adapts links as needed. It generates links to index.html, while this is not needed. It could generate links to the directories that contain these files. This leads to nicer URLs for webpages in the browser. I've seen people share links in papers with index.html, which doesn't look as short and nice as could be.