chore: update CHANGELOG for v0.31.0 (#4044)

Update CHANGELOG for v0.31.0 and remove .md files in doc/changelog.d.

> [!CAUTION]
> Do not merge this pull request until the release has been successfully
made.

---------

Co-authored-by: Prithwish Mukherjee <[email protected]>

Author: pyansys-ci-bot

doc/changelog.d/4044.maintenance.md

@@ -0,0 +1 @@
+update CHANGELOG for v0.31.0

doc/source/changelog.rst

@@ -9,8 +9,9 @@ This document contains the release notes for the project.
 
 .. towncrier release notes start
 
+`0.31.0 <https://github.com/ansys/pyfluent/releases/tag/v0.31.0>`_ - May 21, 2025
+
 `0.32.dev0 <https://github.com/ansys/pyfluent/releases/tag/v0.32.dev0>`_ - May 22, 2025
-=======================================================================================
 
 .. tab-set::
 
@@ -123,7 +124,6 @@ This document contains the release notes for the project.
         * - Fix datatype in field_level_help.csv [skip tests]
           - `#4029 <https://github.com/ansys/pyfluent/pull/4029>`_
 
-
   .. tab-item:: Test
 
     .. list-table::