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/runbooks/index.md

@@ -0,0 +1,17 @@
+# Runbooks
+
+Operational how-tos for recurring tasks — sanitized for public reference.
+
+!!! note "Work in progress"
+    Individual runbooks will be added under this section and wired into the nav.
+
+## Sanitization reminder
+
+Runbooks often touch real infrastructure. Before a runbook goes public:
+
+- Replace real hostnames, internal IPs, and exact topology with placeholders
+  (`<host>`, `<dev>`, `/srv/...`).
+- Strip secrets and credentials entirely — reference where they live, never the
+  values.
+- Keep the procedure and the reasoning; drop the specifics that identify our
+  boxes.