Fix ReadTheDocs 'latest' build (erf-model#1292)

* Explicitly set html_theme_path

* Add sphinx_rtd_theme extension

* Add requirements.txt

* Cleanup

.readthedocs.yml

@@ -5,10 +5,14 @@ build:
   tools:
     python: "3.11"
 
+python:
+  install:
+     - requirements: Docs/sphinx_doc/requirements.txt
+
 sphinx:
    configuration: Docs/sphinx_doc/conf.py
 
 formats:
   - htmlzip
 #  - pdf
-#  - epub
+#  - epub

Docs/sphinx_doc/conf.py

@@ -30,7 +30,9 @@
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = [ 'sphinx.ext.mathjax']
+extensions = [
+    'sphinx.ext.mathjax',
+]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -134,7 +136,9 @@
 # html_theme_options = {}
 
 # Add any paths that contain custom themes here, relative to this directory.
-# html_theme_path = []
+# Add path to the RTD explicitly to robustify builds
+import sphinx_rtd_theme
+html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
 
 # The name for this set of Sphinx documents.
 # "<project> v<release> documentation" by default.

Docs/sphinx_doc/requirements.txt

@@ -0,0 +1 @@
+sphinx-rtd-theme>=1.0.0