Fix Read the Docs build: install sphinx before running sphinx-apidoc

- Add 'pip install sphinx' before sphinx-apidoc command
- Fixes error: 'sphinx-apidoc: not found'
- Build commands run before python.install, so sphinx must be installed explicitly

Author: acul71

.readthedocs.yml

@@ -5,6 +5,7 @@ build:
   tools:
     python: "3.10"
   commands:
+    - pip install sphinx
     - sphinx-apidoc -o docs/ --separate --no-toc -f cid
 
 sphinx: