Doc: Asciidoctor: switch to compat mode for, e.g., single-quote emphasis

...with Asciidoctor.

See also:
https://asciidoctor.org/docs/user-manual/#changed-syntax
https://github.com/asciidoctor/asciidoctor/pull/1004

As long as AsciiDoc Python implementation needs to be supported
(while not receiving any Asciidoctor-compatible modernizing, which is
improbable though possible), there's hardly anything better we can do

• at least make processing through Asciidoctor fully working.