move away from asciidoc

asciidoc is the only part of the tor build process that requires python 2. python 2 is deprecated upstream and is approaching end-of-life.

Over in https://bugs.debian.org/892830 , i asked about upgrading asciidoc to move to python3, but apparently asciidoc itself is on life support, and upstream is encouraging everyone to move to asciidoctor instead (which introduces a ruby dependency) or to use something else like pandoc.

in any case, presumably we want the documentation to stay buildable, so i encourage switching away from asciidoc.