rst: add support for markdown tables (#15854)

* rst: add support for markdown tables

* change template into proc

* don't create unnecessary `seq[string]`

compiler/docgen.nim

@@ -127,6 +127,7 @@ template declareClosures =
     of meCannotOpenFile: k = errCannotOpenFile
     of meExpected: k = errXExpected
     of meGridTableNotImplemented: k = errGridTableNotImplemented
+    of meMarkdownIllformedTable: k = errMarkdownIllformedTable
     of meNewSectionExpected: k = errNewSectionExpected
     of meGeneralParseError: k = errGeneralParseError
     of meInvalidDirective: k = errInvalidDirectiveX