rst: single backticks now render correctly in both rst2html and github (#17028)

* rst: `` => `
* support default-role in rst2html
* update docstyle regarding single vs double backticks

lib/packages/docutils/rstast.nim

@@ -65,6 +65,7 @@ type
     rnInlineTarget,           # "_`target`"
     rnSubstitutionReferences, # "|"
     rnSmiley,                 # some smiley
+    rnDefaultRole,            # .. default-role:: code
     rnLeaf                    # a leaf; the node's text field contains the
                               # leaf val