[doc] reformat changes files headers
to remove the "what's new" phrasing and add the publication date instead.
Related to #4832808
--- a/doc/changes/3.14.rst Fri Jul 03 12:07:29 2015 +0200
+++ b/doc/changes/3.14.rst Fri Jul 03 12:18:56 2015 +0200
@@ -1,5 +1,5 @@
-Whats new in CubicWeb 3.14
-==========================
+3.14 (09/11/2011)
+=================
First notice CW 3.14 depends on yams 0.34 (which is incompatible with prior
cubicweb releases regarding instance re-creation).
--- a/doc/changes/3.15.rst Fri Jul 03 12:07:29 2015 +0200
+++ b/doc/changes/3.15.rst Fri Jul 03 12:18:56 2015 +0200
@@ -1,5 +1,5 @@
-What's new in CubicWeb 3.15?
-============================
+3.15 (12/04/2012)
+=================
New functionnalities
--------------------
--- a/doc/changes/3.16.rst Fri Jul 03 12:07:29 2015 +0200
+++ b/doc/changes/3.16.rst Fri Jul 03 12:18:56 2015 +0200
@@ -1,5 +1,5 @@
-What's new in CubicWeb 3.16?
-============================
+3.16 (25/01/2013)
+=================
New functionalities
--------------------
--- a/doc/changes/3.17.rst Fri Jul 03 12:07:29 2015 +0200
+++ b/doc/changes/3.17.rst Fri Jul 03 12:18:56 2015 +0200
@@ -1,5 +1,5 @@
-What's new in CubicWeb 3.17?
-============================
+3.17 (02/05/2013)
+=================
New functionalities
--------------------
--- a/doc/changes/3.18.rst Fri Jul 03 12:07:29 2015 +0200
+++ b/doc/changes/3.18.rst Fri Jul 03 12:18:56 2015 +0200
@@ -1,5 +1,5 @@
-What's new in CubicWeb 3.18?
-============================
+3.18 (10/01/2014)
+=================
The migration script does not handle sqlite nor mysql instances.
--- a/doc/changes/3.19.rst Fri Jul 03 12:07:29 2015 +0200
+++ b/doc/changes/3.19.rst Fri Jul 03 12:18:56 2015 +0200
@@ -1,5 +1,5 @@
-What's new in CubicWeb 3.19?
-============================
+3.19 (28/04/2015)
+=================
New functionalities
--------------------
--- a/doc/changes/3.20.rst Fri Jul 03 12:07:29 2015 +0200
+++ b/doc/changes/3.20.rst Fri Jul 03 12:18:56 2015 +0200
@@ -1,5 +1,5 @@
-What's new in CubicWeb 3.20
-===========================
+3.20 (06/01/2015)
+=================
New features
------------
--- a/doc/changes/3.21.rst Fri Jul 03 12:07:29 2015 +0200
+++ b/doc/changes/3.21.rst Fri Jul 03 12:18:56 2015 +0200
@@ -1,5 +1,5 @@
-What's new in CubicWeb 3.21?
-============================
+3.21
+====
New features
------------