Merge branch 'sg/asciidoctor-in-ci'

Update our support to format documentation in the CI environment,
either with AsciiDoc ro Asciidoctor.

* sg/asciidoctor-in-ci:
  ci: fix AsciiDoc/Asciidoctor stderr check in the documentation build job
  ci: stick with Asciidoctor v1.5.8 for now
  ci: install Asciidoctor in 'ci/install-dependencies.sh'
  Documentation/technical/protocol-v2.txt: fix formatting
  Documentation/technical/api-config.txt: fix formatting
  Documentation/git-diff-tree.txt: fix formatting