docs: add Google-style docstrings and Sphinx autodoc setup

Add concise docstrings to all Python modules, classes, and public
functions. Configure Sphinx with napoleon extension for Google-style
docstring parsing and autodoc pages for each module.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/source/conf.py

@@ -0,0 +1,24 @@
+import os
+import sys
+
+sys.path.insert(0, os.path.abspath("../.."))
+
+project = "ai_demo_backend"
+copyright = "2026, Mathew Guest"
+author = "Mathew Guest"
+
+extensions = [
+    "sphinx.ext.autodoc",
+    "sphinx.ext.viewcode",
+    "sphinx.ext.napoleon",
+]
+
+templates_path = ["_templates"]
+exclude_patterns = []
+
+html_theme = "sphinx_rtd_theme"
+html_static_path = ["_static"]
+
+autodoc_member_order = "bysource"
+napoleon_google_docstring = True
+napoleon_numpy_docstring = False