Change directory layout and include:: macro calls in a way which is compatible with Gitbook.io

[xtaran]

Currently Gitbook.io doesn't support Asciidoc's include:: macros, see GitbookIO/gitbook#1015. The document structure needs to be defined manually in a file called SUMMARY.adoc with an ordered and nested list containing link: macros. We can generate that SUMMARY.adoc from our current structure by finding all occurrences of include:: macros. The script bin/generate-gitbook-summary.pl in the gitbook branch does that already.

Gitbook.io seems to ignore all files which don't generate content already, i.e. those only containing more than just include:: macros (e.g. a header) are included. But if content and include:: macros are mixed, we get ugly hyperlinks labed with the .adoc file name despite pointing to the right .html file (in the online book). We should be able to avoid them by splitting up all files with such mixed contents.