Pin versions of sphinx and optional extensions

As recommended for reproducible builds on readthedocs:
https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
and to ensure that our docs build works with readthedocs changes to default
project dependencies:
https://blog.readthedocs.com/defaulting-latest-build-tools/

envs/environment-dev.yaml

@@ -92,9 +92,9 @@ dependencies:
   - tomli
 
   # For documentation
-  - sphinx
-  - sphinx_rtd_theme=1.2
-  - sphinx-notfound-page
+  - sphinx=5.3.0
+  - sphinx-notfound-page=1.0.0
+  - sphinx-rtd-theme=1.3.0
 
   # For exploration
 #  - ipython

envs/environment-fig-dev.yaml

@@ -69,9 +69,9 @@ dependencies:
   - jupyterlab
 
   # For documentation
-  - sphinx
-  - sphinx_rtd_theme=1.2
-  - sphinx-notfound-page
+  - sphinx=5.3.0
+  - sphinx-notfound-page=1.0.0
+  - sphinx-rtd-theme=1.3.0
 
   - pip:
     - angles

envs/environment-linkcheck.yaml

@@ -66,9 +66,9 @@ dependencies:
   - xarray
 
   # For documentation links checking
-  - sphinx
-  - sphinx_rtd_theme=1.2
-  - sphinx-notfound-page
+  - sphinx=5.3.0
+  - sphinx-notfound-page=1.0.0
+  - sphinx-rtd-theme=1.3.0
 
   - pip:
     - angles

envs/environment-rtd.yaml

@@ -15,10 +15,19 @@ dependencies:
   - python=3.12
   - retrying
 
-  # RTD packages
+  # Sphinx and extensions
+  - sphinx=5.3.0
+  - sphinx-notfound-page=1.0.0
+  - sphinx-rtd-theme=1.3.0
+
+  # readthedocs build system packages
   - mock
   - pillow
   - pip:
+    - commonmark
+    - recommonmark
+    - readthedocs-sphinx-ext
+
     # Install NEMO_Nowcast and SalishSeaNowcast packages in editable mode
     - --editable git+https://github.com/43ravens/NEMO_Nowcast.git#egg=NEMO_Nowcast
     - --editable ../