add github workflow to generate html and pdf artifacts

[ahouseholder]

resolves #184

This PR includes the following changes:

• a github workflow that will produce an HTML and PDF artifact on each push to main or develop branches that affects any file in the doc directory or the workflow file itself.
• adds a Makefile that replaces previous shell scripts for manually generating HTML and PDF locally (converting these scripts to a Makefile helped with figuring out the structure of the github workflow)
  • sample output from the Makefile can be found in ssvc_v2-0.html and ssvc_v2-0.pdf
• Updates some of the "how-to" files in the doc directory to markdown format

Note that the github workflow uploads its artifacts to the workflow summary page rather than modifying the repository directly.

This PR is a squashed version of my current develop branch, so if you want to see a sample of the output of the workflow you can check out https://github.com/ahouseholder/SSVC/actions/runs/4834190249

[sei-vsarvepalli]

Wow - Good work @ahouseholder. I will follow-up with Dryad (ssvc-calc folder) updates to do the same and publish these here instead of democert.org using GitHub Pages.