Improve output tables using {flextable}, {gt}, and others

Motivated by the following blog post: https://themockup.blog/posts/2020-09-04-10-table-rules-in-r/

{gt} now appears to support HTML, Latex, and RTF outputs, so SHOULD work with the output from all .Rmd files and avoid the need for conditional outputs based on document type and therefore simplify coding.