use conda to get more recent nodejs

RTD nodejs is too old, and conda install appears to be the only way to get a bump

Author: minrk

docs/environment.yml

@@ -0,0 +1,16 @@
+name: ipyparallel-docs
+channels:
+  - conda-forge
+dependencies:
+  - python=3.8
+  - pip
+
+  - sphinx>=1.8
+  - sphinx-copybutton
+  - pydata-sphinx-theme
+  - nbsphinx
+  - myst-parser
+
+  - matplotlib
+  # nodejs needed for extension build
+  - nodejs

readthedocs.yml

@@ -3,12 +3,16 @@ version: 2
 sphinx:
   configuration: docs/source/conf.py
 
+conda:
+  environment: docs/environment.yml
+
 build:
   image: latest
 
 python:
   version: 3.8
   install:
+    # install ipp itself
     - method: pip
       path: .
     - requirements: docs/requirements.txt