Added site.baseurl for cases where site is served from sub-directory

[huyen-nguyen]

• Added baseurl as in {{ site.url }}{{ site.baseurl }} to prepend to the url.
• url and baseurl are specified in _config.yml, where:
  • url: Root of the domain, including protocol, domain, and port (if applicable), e.g., https://example.com
  • baseurl: Optional. Name of sub-directory where the site is served from, e.g., /Lagrange in https://example.com/Lagrange