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/libraries/index.md

@@ -0,0 +1,23 @@
+# Libraries
+
+Usage docs for the shared `rethink-public` library suite. Before hand-rolling
+common functionality, check the suite first and prefer an existing lib.
+
+!!! note "Work in progress"
+    A page per library will be added here. Browse the `rethink-public` org for
+    the live list — each repo's tagline says what it does.
+
+## Install pattern
+
+```
+<lib> @ git+https://git.rethinkstudios.io/rethink-public/<lib>.git@<tag>
+```
+
+## Common capabilities covered by the suite
+
+Retry/backoff, logging setup, HTTP sessions, proxies, webhooks, IMAP/mail,
+Mongo/KV/datastore, crypto/envelope, `discord.py` helpers, and timing/paths/
+masking utilities.
+
+If a capability isn't in the suite, build it in-project — and flag whether it's
+generic enough to become a new shared lib.