Update Jekyll example to upgrade Jekyll version, fix build errors and clean up files

[MichaelCurrin]

Main changes:

• Update to Jekyll 3.9 for security and add kramdown parser as it is necessary.
• Add vendor/ to ignore file
• Add missing gems in Gemfile to build file to avoid errors in using tags
• Add Makefile for convenience - e.g.

  make install
  make serve
  

• Improve install, usage and configure instructions
• Add tailwind.config.js to version control so it can be used on remote builds. Move the setup note to the end of the README.md
• Cleaned up config file. Improved the excludes list to add items and remove .name files which are automatically ignored
• Fix CSS import
• Rename site.css to styles.css and referenced it with link tag in head.html for safety