rtd with latest mambaforge image for faster building

.readthedocs.yml

@@ -1,20 +1,20 @@
 version: 2
 
 build:
-    image: latest
+  os: ubuntu-20.04
+  tools:
+    python: mambaforge-4.10
 
 conda:
-    environment: requirements/ci/readthedocs.yml
+  environment: requirements/ci/readthedocs.yml
 
 sphinx:
-    configuration: docs/src/conf.py
-    fail_on_warning: false
+  configuration: docs/src/conf.py
+  fail_on_warning: false
 
 python:
-    install: 
-        - method: setuptools
-          path: .
-
-formats:
-    - htmlzip
-    - pdf
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs

docs/src/whatsnew/latest.rst

@@ -200,6 +200,9 @@ This document explains the changes made to Iris for this release
 #. `@wjbenfold`_ clarified behaviour of :func:`iris.load` in :ref:`userguide loading section
    <loading_iris_cubes>`. (:pull:`4462`)
 
+#. `@bjlittle`_ migrated readthedocs to use mambaforge for `faster documentation building`_.
+   (:pull:`4476`)
+
 
 💼 Internal
 ===========
@@ -273,3 +276,4 @@ This document explains the changes made to Iris for this release
 .. _NEP-29: https://numpy.org/neps/nep-0029-deprecation_policy.html
 .. _UGRID: http://ugrid-conventions.github.io/ugrid-conventions/
 .. _sort-all: https://github.com/aio-libs/sort-all
+.. _faster documentation building: https://docs.readthedocs.io/en/stable/guides/conda.html#making-builds-faster-with-mamba