Try to reconcile README, Asciidoc and GitHub (v3)

[jimklimov]

Closes: #226
Closes: #669
Related-to: #1953 and #2048

Includes earlier attempts from https://github.com/aquette/nut/tree/gh_adoc, https://github.com/zykh/nut/tree/rename_readme and https://github.com/clepple/nut/tree/rename_readme - thanks to @zykh for figuring out much of the magic behind the scenes, and @clepple and @aquette for earlier blueprints!

Also, note that the proposed solution is rather clumsy due to github/markup#1095 (lack of custom asciidoc macros, configuration files, and include:...[] pre-processing) in the GitHub Web-UI renderer - so the least-bad approach, maintainable and working, was to define the needed asciidoc attributes in a new docs/asciidoc-vars.conf file and source it into original documents that use these multi-targeted features with make maintainer-asciidocs. If GH eventually solves their end, or at least some layers of it, the clumsy part in NUT sources and recipes can be easily retracted:

• use of docs/asciidoc-vars.conf file might become a run-time include (anticipated syntax commented in README.adoc
• vars themselves might not be needed and macros would be usable, for a less confusing syntax than currently with chains of prefix, separator and suffix expandable variables (attributes).

This change currently also renders cross-reference links from one NUT document to potentially another's section as URLs to the published NUT web site, not as links to some neighboring document filename (not easy to guess statically) and hash-anchors there, hosted in the same Git branch, which would have been more helpful.

Checked to render reasonably while browsing the NUT repo on GitHub (with links pointing to expected-looking locations on the NUT website, even if they have not been published there yet - chicken-and-egg problem), as well as PDF renders (e.g. user-manual.pdf has internal cross-section links as well as external ones pointing to the NUT website or other "nearby" PDFs).

Further work in this area should include:

• Conversion of other major docs to *.adoc and the asciidoc-vars.conf hacks, if only to make their maintenance consistent; possibly the MAN page sources can be exempt as an island of their own (not so many links and includes to worry about, and original asciidoc is more readable as source there even on github, not too much readability benefit from pretty rendering);
• Revise cross-references in those docs (convert to variables where needed, to cover different target media);
• Consider ifdef ... choice of link types, as used already in some documents now for website/txt builds - to use http (or xref) links to document sections vs. pointers to individual filenames (the latter may help with GitHub source browsing, to auto-point to doc files in the same commit or branch under development);
• Consider embedding default top_srcdir and top_builddir (as respective {srcdir}/.. etc.) into newly christened *.adoc sources based on their location in codebase (. for workspace root, .. for docs/, lib/ and such, ../../ for docs/man, scripts/something, etc.)