Implement Markdown definition lists (+ migration) (#20333)

Implements definition lists Markdown extension adopted in a few
implementations including:
* [Pandoc](
  https://pandoc.org/MANUAL.html#definition-lists)
* [kramdown](
  https://kramdown.gettalong.org/quickref.html#definition-lists)
* [PHP extra Markdown](
  https://michelf.ca/projects/php-markdown/extra/#def-list)

Also affected files have been migrated.
RST definition lists are turned off for Markdown: this solves the
problem of broken formatting mentioned in
https://github.com/nim-lang/Nim/pull/20292.

lib/packages/docutils/rstgen.nim

@@ -1212,7 +1212,7 @@ proc renderRstToOut(d: PDoc, n: PRstNode, result: var string) =
   of rnBulletItem, rnEnumItem:
     renderAux(d, n, "<li$2>$1</li>\n", "\\item $2$1\n", result)
   of rnEnumList: renderEnumList(d, n, result)
-  of rnDefList:
+  of rnDefList, rnMdDefList:
     renderAux(d, n, "<dl$2 class=\"docutils\">$1</dl>\n",
                     "\\begin{description}\n$2\n$1\\end{description}\n", result)
   of rnDefItem: renderAux(d, n, result)