Added Sphinx based documentation

This patch adds the docuemntation source tree in docs.

You can build the documentation with

tox -edocs

View the documentation by opening a browser and viewing
aprsd/docs/_build/index.html

docs/clean_docs.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python3
+
+"""Removes temporary Sphinx build artifacts to ensure a clean build.
+
+This is needed if the Python source being documented changes significantly. Old sphinx-apidoc
+RST files can be left behind.
+"""
+
+from pathlib import Path
+import shutil
+
+
+def main() -> None:
+    docs_dir = Path(__file__).resolve().parent
+    for folder in ("_build", "apidoc"):
+        delete_dir = docs_dir / folder
+        if delete_dir.exists():
+            shutil.rmtree(delete_dir)
+
+
+if __name__ == "__main__":
+    main()