Build on Read the Docs

Use new `build.os` and `build.tools` configs from the `.readthedocs.yaml` config
file to use Ubuntu 20.04 LTS and install NodeJS v14.

Run `npx gulp build` from inside `conf.py` before building documentation with
Sphinx.

.readthedocs.yaml

@@ -0,0 +1,17 @@
+version: 2
+
+build:
+  os: "ubuntu-20.04"
+  tools:
+    python: "3.10"
+    nodejs: "14"
+
+sphinx:
+  configuration: docs/conf.py
+
+python:
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - doc

docs/conf.py

@@ -4,6 +4,7 @@
 # https://www.sphinx-doc.org/en/master/usage/configuration.html
 
 import os
+import subprocess
 import sys
 
 # add the demo python code to the path, so that it can be used to demonstrate
@@ -87,3 +88,10 @@
         "</a>!"
     )
 }
+
+# Generate JS/CSS assets before running Sphinx
+subprocess.run([
+    'npx',
+    'gulp',
+    'build',
+])