Fix many broken links

Note that contrary to what docgen.rst currently says, the ids have
to match exactly or else most web browsers will not jump to the
intended symbol.

lib/pure/concurrency/threadpool.nim

@@ -487,7 +487,7 @@ proc parallel*(body: untyped) {.magic: "Parallel".}
   ##
   ## ``body`` has to be in a DSL that is a particular subset of the language.
   ##
-  ## Please refer to `the manual <manual.html#parallel-amp-spawn>`_
+  ## Please refer to `the manual <manual_experimental.html#parallel-amp-spawn>`_
   ## for further information.
 
 var