docs: link to 7.5 relnotes

docs/relnotes.html

@@ -20,6 +20,7 @@ The release notes summarize what's new or changed in each Mesa release.
 </p>
 
 <UL>
+<LI><A HREF="relnotes-7.5.html">7.5 release notes</A>
 <LI><A HREF="relnotes-7.4.2.html">7.4.2 release notes</A>
 <LI><A HREF="relnotes-7.4.1.html">7.4.1 release notes</A>
 <LI><A HREF="relnotes-7.4.html">7.4 release notes</A>