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.

doc/tools.rst

@@ -20,7 +20,7 @@ The standard distribution ships with the following tools:
 - | `C2nim <https://github.com/nim-lang/c2nim/blob/master/doc/c2nim.rst>`_
   | C to Nim source converter. Translates C header files to Nim.
 
-- | `niminst <https://nim-lang.org/docs/niminst.html>`_
+- | `niminst <niminst.html>`_
   | niminst is a tool to generate an installer for a Nim program.
 
 - | `nimgrep <nimgrep.html>`_