release: Zephyr 3.1.0

Update versions for release v3.1.0.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
diff --git a/VERSION b/VERSION
index 667e868..d8558ba 100644
--- a/VERSION
+++ b/VERSION
@@ -2,4 +2,4 @@
 VERSION_MINOR = 1
 PATCHLEVEL = 0
 VERSION_TWEAK = 0
-EXTRAVERSION = rc3
+EXTRAVERSION = 0
diff --git a/doc/conf.py b/doc/conf.py
index def2331..662cd2d 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -140,6 +140,7 @@
     "current_version": version,
     "versions": (
         ("latest", "/"),
+        ("3.1.0", "/3.1.0/"),
         ("3.0.0", "/3.0.0/"),
         ("2.7.0", "/2.7.0/"),
         ("2.6.0", "/2.6.0/"),