RST: implement footnotes and citations (#16960)

* RST: implement footnotes and citations
* manual fixup of nimdoc.out.css
* remove unused code
* shorter printing code
* Update lib/packages/docutils/rst.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

compiler/docgen.nim

@@ -137,6 +137,7 @@ template declareClosures =
     of meNewSectionExpected: k = errNewSectionExpected
     of meGeneralParseError: k = errGeneralParseError
     of meInvalidDirective: k = errInvalidDirectiveX
+    of meFootnoteMismatch: k = errFootnoteMismatch
     of mwRedefinitionOfLabel: k = warnRedefinitionOfLabel
     of mwUnknownSubstitution: k = warnUnknownSubstitutionX
     of mwUnsupportedLanguage: k = warnLanguageXNotSupported