docs: move old relnotes to _extra directory

These aren't built, so they won't get copied out into the public folder
unless we put is inside the _extra folder.

This has been broken all since the conversion to Sphinx, whoops!

Reviewed-by: Eric Engestrom <eric@igalia.com>
Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21956>

docs/_extra/relnotes/3.4.2

@@ -0,0 +1,21 @@
+
+                         Mesa 3.4.2 release notes
+
+                               May 17, 2001
+
+                              PLEASE READ!!!!
+
+
+
+Introduction
+------------
+
+Mesa uses an even/odd version number scheme like the Linux kernel.
+Odd numbered versions (such as 3.3) designate new developmental releases.
+Even numbered versions (such as 3.4) designate stable releases.
+
+Mesa 3.4.2 is a maintenance release that simply fixes bugs found since
+the Mesa 3.4.1 release.  For details, see the VERSIONS file.
+
+
+----------------------------------------------------------------------