Prefer relative links for Nim documentation

This is more friendly to those browsing the documentation without
a network connection. The nim-doc package in Debian allows this,
for example.

Also, the domain name being used was not consistent. It could have
been either nim-lang.org or nim-lang.github.io, and those reading
the stable docs could have found themselves suddenly reading the
devel docs instead.

lib/core/macros.nim

@@ -13,8 +13,8 @@ include "system/inclrtl"
 ## tree (`AST`:idx:). Macros operate on this tree.
 ##
 ## See also:
-## * `macros tutorial <https://nim-lang.github.io/Nim/tut3.html>`_
-## * `macros section in Nim manual <https://nim-lang.github.io/Nim/manual.html#macros>`_
+## * `macros tutorial <tut3.html>`_
+## * `macros section in Nim manual <manual.html#macros>`_
 
 ## .. include:: ../../doc/astspec.txt