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.

config.toml.example

@@ -0,0 +1,22 @@
+# Local development configuration for `just dev`.
+# Copy to config.toml and adjust. Keep real admin hashes out of Git.
+
+[server]
+host = "127.0.0.1"
+port = 9090
+
+content_dir = "./posts"
+users_file = "./users.toml"
+
+[site]
+title = "volumen (dev)"
+description = "Local development instance of volumen."
+base_url = "http://localhost:9090"
+language = "cs"
+author = "Petr Balvín"
+
+[admin]
+# Generate a hash with: bundle exec exe/volumen hash-password
+password_hash = ""
+session_key = ""
+session_ttl = 86400