doc tool - support GitHub flavoured markdown table

[bishabosha]

see here https://slinc.hammons.fr/docs/docs/reference/usage.html

there is already a standard extension for flexmark (which we use to render the markdown) so this would be a few lines change.

[SethTisue]

prior art in Scala 2:

• https://www.scala-lang.org/blog/2018/10/04/scaladoc-tables.html
• https://github.com/scala/scala/pull/6043

note the section of the blog post about eventual Scala 3 support :-)