scaffold MkDocs Material handbook site

Initialize the public Rethink Studios docs site:
- mkdocs.yml: Material theme, light/dark palette toggle, pymdownx
  extensions, search, four-section nav skeleton.
- docs/index.md landing page with a card grid.
- Stub index pages for deploy, conventions, libraries, runbooks.
- .gitignore excludes .claude/, site/, and Python build cruft.

Deploy guide deferred; deploy section is a placeholder for now.

Signed-off-by: disqualifier <dev@disqualifier.me>

docs/conventions/index.md

@@ -0,0 +1,19 @@
+# Conventions
+
+Coding, library, and infrastructure conventions that keep projects consistent.
+
+!!! note "Work in progress"
+    Individual convention pages will be added under this section and wired into
+    the nav.
+
+## Topics to document
+
+- **Python style** — flake8 clean (max line 120), type hints on public
+  functions, lowercase-start docstrings with no trailing period.
+- **Library layout** — `src/` layout, hatchling, Python ≥ 3.10, underscores in
+  names, `aio`-prefix only for libs that define async surfaces.
+- **Logging** — emit-only in libraries (`logging.getLogger(__name__)`); handler,
+  level, and format configured only at the application entry point.
+- **Files** — trailing newline, no trailing whitespace, LF line endings.
+- **Config** — config-free modules (inject dependencies), single source of
+  truth, no duplicated logic or constants.