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.

doc/lib.rst

@@ -329,7 +329,7 @@ Threading
 ---------
 
 * `threadpool <threadpool.html>`_
-  Implements Nim's `spawn <manual.html#parallel-amp-spawn>`_.
+  Implements Nim's `spawn <manual_experimental.html#parallel-amp-spawn>`_.
 
 
 Parsers