Fix some asciidoc cross-file links

[jimklimov]

• Fix the linkdoc macro in chunked-html (now that we generate chunk file names not numbered, but using their anchor name)
• Add macros to wrap references to source files as such
  • Feels like more can be done here, e.g. for github rendering, or to refer to online version of that source from published docs
• Allow a third argument to linkdoc and linksingledoc to generate a reference to file name in sources (for rapidly evolving ones)
• Add a few samples for new use-cases in existing docs
  • If this seems like the way to do it, should be applied to countless other cases in docs sources
• Fix older markup issues in older docs
• Update resources used in NUT CI rendering (CSS)

[AppVeyorBot]

✅ Build nut 2.8.2.2284-master completed (commit 110490be92 by @jimklimov)