Use a different requirements file for the docs to deal with cartopy

.readthedocs.yaml

@@ -34,4 +34,4 @@ python:
   install:
     - method: pip
       path: .
-    - requirements: requirements.txt
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,18 @@
+alive_progress
+astropy
+#cartopy
+cdasws
+configparser
+h5py
+jupyterlab
+matplotlib
+pandas
+progressbar
+pyhdf
+pyspedas
+pytest
+slack_sdk
+spacepy
+sphinx-rtd-theme
+sphinxcontrib-autoprogram
+sunpy