Whats new updates for v3.9.0rc0 (#5899)

* Whats new updates for v3.9.0rc0 .

* Update docs/src/whatsnew/3.9.rst

Co-authored-by: Patrick Peglar <[email protected]>

---------

Co-authored-by: Patrick Peglar <[email protected]>

Author: trexfeathers

Diff for: docs/src/whatsnew/latest.rst renamed to docs/src/whatsnew/3.9.rst

@@ -1,21 +1,26 @@
 .. include:: ../common_links.inc
 
-|iris_version| |build_date| [unreleased]
-****************************************
+v3.9 (03 Apr 2024) [release candidate]
+**************************************
 
 This document explains the changes made to Iris for this release
 (:doc:`View all changes <index>`.)
 
 
-.. dropdown:: |iris_version| Release Highlights
+.. dropdown:: v3.9 Release Highlights
    :color: primary
    :icon: info
    :animate: fade-in
    :open:
 
-   The highlights for this major/minor release of Iris include:
+   This is a small release to make two important changes available as soon as
+   possible:
 
-   * N/A
+   * The :mod:`iris.experimental.geovista` module.
+   * Removal of fill value collision warnings in NetCDF saving, which
+     significantly improves Iris' performance when parallel processing.
+
+   See below for more detail on these changes.
 
    And finally, get in touch with us on :issue:`GitHub<new/choose>` if you have
    any issues or feature requests for improving Iris. Enjoy!

Diff for: docs/src/whatsnew/index.rst

@@ -1,17 +1,18 @@
+
 .. include:: ../common_links.inc
 
 .. _iris_whatsnew:
 
 What's New in Iris
 ------------------
 
-.. include:: latest.rst
+.. include:: 3.9.rst
 
 .. toctree::
    :maxdepth: 1
    :hidden:
 
-   latest.rst
+   3.9.rst
    3.8.rst
    3.7.rst
    3.6.rst