This allows creating HTML presentations with
`asciidoctor-revealjs presentation.adoc`.
NOTE: The generated HTML file implicitly depend on reveal.js (and I see
no good way to bundle it with Nix). Either provide reveal.js in a local
path next to the generated HTML file[1], or override the revealjsdir
attribute, for example by pointing to an URL (can also be a path):
asciidoctor-revealjs -a revealjsdir=https://cdn.jsdelivr.net/npm/reveal.js@3.9.2 presentation.adoc
Implementation details:
1. Added "gem 'asciidoctor-revealjs'" to the Gemfile.
2. Ran "nix-shell -p bundler --run 'bundle lock --update'"
from pkgs/tools/typesetting/asciidoctor/.
3. Hand edited Gemfile.lock to remove all but the asciidoctor-revealjs
changes. (Rationale: allow backporting to release-20.09.)
4. Finish off with "nix-shell -p bundix --run 'bundix'".
[1] Of course Nix can help with _that_, but that's external to the
asciidoctor-revealjs program.
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