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

compiler/lineinfos.nim

@@ -34,6 +34,7 @@ type
     errGeneralParseError,
     errNewSectionExpected,
     errInvalidDirectiveX,
+    errFootnoteMismatch,
     errProveInit, # deadcode
     errGenerated,
     errUser,
@@ -84,6 +85,7 @@ const
     errGeneralParseError: "general parse error",
     errNewSectionExpected: "new section expected",
     errInvalidDirectiveX: "invalid directive: '$1'",
+    errFootnoteMismatch: "number of footnotes and their references don't match: $1",
     errProveInit: "Cannot prove that '$1' is initialized.",  # deadcode
     errGenerated: "$1",
     errUser: "$1",