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/5.0.2

@@ -0,0 +1,45 @@
+
+                           Mesa 5.0.2 release notes
+
+                               September 5, 2003
+
+
+
+Introduction
+------------
+
+Mesa uses an even/odd version number scheme like the Linux kernel.
+Even-numbered versions (such as 5.0.x) designate stable releases.
+Odd-numbered versions (such as 4.1.x) designate new developmental releases.
+
+Mesa 5.0.2 just fixes bugs found since the 5.0.1 release.  See the VERSIONS
+file for details.
+
+
+Device Driver Status
+--------------------
+
+A number of Mesa's software drivers haven't been actively maintained for
+some time.  We rely on volunteers to maintain many of these drivers.
+Here's the current status of all included drivers:
+
+
+Driver			Status
+----------------------	---------------------
+XMesa (Xlib)		implements OpenGL 1.4
+OSMesa (off-screen)	implements OpenGL 1.4
+FX (3dfx Voodoo1/2)	implements OpenGL 1.3
+SVGA			implements OpenGL 1.3
+Wind River UGL		implements OpenGL 1.3
+Windows/Win32		implements OpenGL 1.4
+DJGPP			implements OpenGL 1.4
+GGI			implements OpenGL 1.3
+BeOS			implements OpenGL 1.4
+Allegro			needs updating
+D3D			needs updating
+
+Note: supporting OpenGL 1.4 (vs. 1.3 or 1.2) usually only requires that the
+driver call the _mesa_enable_1_4_extensions() function.
+
+
+----------------------------------------------------------------------