chore: gitignore config.toml, add config.toml.example

The dev config.toml contains a password hash and should not be
committed. Add a config.toml.example template instead and document
the copy step.

.gitignore

@@ -13,6 +13,9 @@
 # Local development sandbox (posts, generated config)
 /.volumen/
 
+# Local dev config (contains password hash)
+/config.toml
+
 # Uploaded media in the dev content directory
 /posts/media/