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/sets_fragment.txt

@@ -83,4 +83,4 @@ Note how the set turns enum values into powers of 2.
 If using enums and sets with C, use distinct cint.
 
 For interoperability with C see also the
-`bitsize pragma <#implementation-specific-pragmas-bitsize-pragma>`_.
+`bitsize pragma <manual.html#implementation-specific-pragmas-bitsize-pragma>`_.