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