Initial setup and configure of Retype

[geoffreymcgill]

Setup and configure Retype static site generator for Markdig.

Based on #439 (comment).

This generated website can be hosted from GitHub Pages, although the repo owner would need to make one simple config to enable the GitHub Pages hosting. Details available at:
https://retype.com/guides/github_actions/#pick-a-branch

The website could also be hosted from a custom domain or subdomain. If that's the case, a couple of minor changes would need to be made to the retype.json. Based on the following docs:
https://retype.com/guides/github_actions/#custom-domain

The following two changes would be required in the retype.json if using custom domain or subdomain hosting with GitHub Pages:

Revised retype.json for custom domain or subdomain

{
  "input": ".",
  "output": ".retype",
  "base": "markdig",          <-- remove base
  "cname": "docs.example.com" <-- add cname
  ...
}

No worries if this is of no interest; feel free to close or delete the PR.

Hope this helps.