docs: restructure release notes into separate folder

relnotes-*html > relnotes/*html
RELNOTES-* > relnotes/*
fix links, css and frames

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
diff --git a/docs/versions.html b/docs/versions.html
index 0047ee0..30c1817 100644
--- a/docs/versions.html
+++ b/docs/versions.html
@@ -1403,8 +1403,8 @@
 <li>demo of per-pixel lighting with a fragment program (demos/fplight.c)
 <li>new version (18) of glext.h header
 <li>new spriteblast.c demo of GL_ARB_point_sprite
-<li>faster glDrawPixels in X11 driver in some cases (see RELNOTES-5.1)
-<li>faster glCopyPixels in X11 driver in some cases (see RELNOTES-5.1)
+<li>faster glDrawPixels in X11 driver in some cases (see relnotes/5.1)
+<li>faster glCopyPixels in X11 driver in some cases (see relnotes/5.1)
 </ul>
 Bug fixes:
 <ul>