Signed-off-by: Stephano Cetola <scetola@linuxfoundation.org>

Building now has 3 options:
1. local build
2. docker container
3. GitHub action

The local build has been tested on macOS Monterey.

The docker container has been tested on Ubuntu 20.04.

In order to use the GitHub action you'll have to use this template to generate your own repo.

Some folks were confused when the result simply said "pdf". This has been changed to "result.pdf" so it is clear that they should click on that link to download the PDF.

Add workflow_dispatch trigger.

This allows manual triggering of the workflow via the github UI.

Enable GitHub actions to generate PDF

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