Add Makefile to specify actions to take in the environment.

Preserve PDF output as an artifact so that it's available to users.

Environment dependencies are recorded in package-manager-specific files
in dependencies/.

Work around issue #8.

Future possible enhancements:
  - Add a release workflow.
  - Move to a container for the environment.  This allows the
    environment description to not be local to github actions, usable
    for local development, etc.

adding note on asciidoctor-mathematical limitations and output to svg for math expressions in build command

The requirements for asciidoctor-mathematical include gdk-pixbuf (spelling fixed) and cmake.