RTD update: Add links to CHANGELOG.md files in versioning.rst

docs/source/references/versioning.rst
- Add links to the CHANGELOG.md files in the science codebase, GCHP
  wrapper, and HEMCO repositories.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

docs/source/reference/versioning.rst

@@ -1,6 +1,16 @@
+.. _gchp-ver:
 
+####################
+GCHP version history
+####################
 
-Versioning
-==========
+For a list of updates by GCHP version, please see:
 
-todo
+- `CHANGELOG.md for the GEOS-Chem science codebase
+  <https://github.com/geoschem/geos-chem/blob/main/CHANGELOG.md>`_
+
+- `CHANGELOG.md for the GCHP wrapper
+  <https://github.com/geoschem/GCHP/blob/main/CHANGELOG.md>`_
+
+- `CHANGELOG.md for HEMCO
+  <https://github.com/geoschem/HEMCO/blob/main/CHANGELOG.md>`_