docs: initial draft of PG 11 release notes

doc/src/sgml/filelist.sgml

@@ -166,6 +166,7 @@
 <!ENTITY sourcerepo SYSTEM "sourcerepo.sgml">
 
 <!ENTITY release    SYSTEM "release.sgml">
+<!ENTITY release-11     SYSTEM "release-11.sgml">
 <!ENTITY release-10     SYSTEM "release-10.sgml">
 <!ENTITY release-9.6    SYSTEM "release-9.6.sgml">
 <!ENTITY release-9.5    SYSTEM "release-9.5.sgml">

doc/src/sgml/release.sgml

@@ -76,6 +76,7 @@ For new features, add links to the documentation sections.
   The reason for splitting the release notes this way is so that appropriate
   subsets can easily be copied into back branches.
 -->
+&release-11;
 &release-10;
 &release-9.6;
 &release-9.5;