docs: link to archived Fedora code of conduct

Fedora has switched to a different CoC.  QEMU's own code of conduct
is based on the previous version and cites it as a source.  Replace
the link with one to the Wayback Machine.

Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: bonzini

docs/devel/code-of-conduct.rst

@@ -55,6 +55,6 @@ Sources
 -------
 
 This document is based on the `Fedora Code of Conduct
-<https://fedoraproject.org/code-of-conduct>`__ and the
-`Contributor Covenant version 1.3.0
+<http://web.archive.org/web/20210429132536/https://docs.fedoraproject.org/en-US/project/code-of-conduct/>`__
+(as of April 2021) and the `Contributor Covenant version 1.3.0
 <https://www.contributor-covenant.org/version/1/3/0/code-of-conduct/>`__.