use Python-only environment for ReadTheDocs builds (#3955)

Co-authored-by: Bouwe Andela <[email protected]>

Author: valeriupredoi

.readthedocs.yaml

@@ -27,7 +27,7 @@ build:
 # Declare the requirements required to build your docs
 conda:
   environment:
-    environment.yml
+    environment_python.yml
 
 # Build documentation in the doc directory with Sphinx
 sphinx:

doc/sphinx/source/quickstart/installation.rst

@@ -364,6 +364,13 @@ so you will not be able to run NCL, R, or Julia diagnostics with it.
     would like to keep.
     It is recommended that you create a new environment when updating ESMValTool.
 
+.. note::
+    There is also a pure-Python environment file ``esmvaltool_python.yml``
+    which is a softlink of the ``environment_osx.yml`` file; this one is used
+    by any build that needs only Python packages (i.e. no NCL and R), currently
+    this is used by our documentation builds, but it could be used by anyone
+    needing just the Python dependencies.
+
 Next, activate the environment by using the command:
 
 .. code-block:: bash

environment_python.yml

@@ -0,0 +1 @@
+./environment_osx.yml