fix #13218: avoid some irrelevant warnings for nim doc,rst2html,--app:lib, + other fixes (#13550)

* fix #13218: avoid some irrelevant warnings for nim doc,rst2html
* suppress warnRedefinitionOfLabel for nim doc
* lots of fixes for UnusedImport warnings

doc/docgen_sample.nim

@@ -4,9 +4,9 @@ import strutils
 
 proc helloWorld*(times: int) =
   ## Takes an integer and outputs
-  ## as many "hello world!"s
+  ## as many indented "hello world!"s
 
   for i in 0 .. times-1:
-    echo "hello world!"
+    echo "hello world!".indent(2) # using indent to avoid `UnusedImport`
 
 helloWorld(5)