Fix sphinx-apidoc command in Read the Docs build (#32)

Install Sphinx first and use python -m sphinx.ext.apidoc instead of
sphinx-apidoc command which is not available in PATH during build commands.

Author: acul71

.readthedocs.yaml

@@ -5,8 +5,9 @@ build:
   tools:
     python: "3.10"
   commands:
+    - python -m pip install sphinx
     - rm -f docs/multibase.rst docs/modules.rst
-    - sphinx-apidoc -o docs/ multibase
+    - python -m sphinx.ext.apidoc -o docs/ multibase
 
 sphinx:
   configuration: docs/conf.py