docs: change old SourceForge link to GitHub

I accidentally followed a very old link and was briefly discombobulated.
To save other people from this mistake, use the current location
of the Meson repository, and tweak the surrounding text so it is
more clear that it was written nearly 10 years ago.

Signed-off-by: Tony Finch <dot@dotat.at>

docs/markdown/Design-rationale.md

@@ -251,10 +251,10 @@ Get the code
 --
 
 The code for this experiment can be found at [the Meson
-repository](https://sourceforge.net/p/meson/code/). It should be noted
-that it is not a build system. It is only a proposal for one. It does
-not work reliably yet. You probably should not use it as the build
-system of your project.
+repository](https://github.com/mesonbuild/meson). It should be noted
+that (at the time of writing) it is not a build system. It is only
+a proposal for one. It does not work reliably yet. You probably
+should not use it as the build system of your project.
 
 All that said I hope that this experiment will eventually turn into a
 full blown build system. For that I need your help. Comments and