CI Processes for spec

[toddbaert]

I'm interested in creating some CI processes for the spec repo. Some functionality I think would be helpful:

• [x] linting markdown, specifically in according with the style-guide (https://www.npmjs.com/package/markdownlint)
• [x] ensuring links between markdown are valid (https://www.npmjs.com/package/markdown-link-check)
• [x] generate the RFC 2119 assertion JSON files automatically (currently manual), and run some basic validation on them.
• [x] validate ToCs?
• [X] JSON file validation (thx @justinabrahms)
• [X] check JSON assertion files has been done (thx @dyladan)

I think we can do all these with some simple packages, enhancements to the Makefile, and github actions.