Switch config format from YAML to TOML

Replaces gopkg.in/yaml.v3 with github.com/BurntSushi/toml. Updates
struct tags, default config path, Dockerfile CMD, README, and ships
config.example.toml in place of the YAML example. ${ENV_VAR}
expansion still runs on the raw file before parsing, so the behavior
is unchanged.

go.mod

@@ -3,7 +3,7 @@ module bellhop
 go 1.25.0
 
 require (
-	gopkg.in/yaml.v3 v3.0.1
+	github.com/BurntSushi/toml v1.6.0
 	maunium.net/go/mautrix v0.28.0
 )