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.

Dockerfile

@@ -18,4 +18,4 @@ COPY --from=build /out/bellhop /usr/local/bin/bellhop
 USER bellhop
 VOLUME ["/app/data"]
 ENTRYPOINT ["/usr/local/bin/bellhop"]
-CMD ["-config", "/app/config.yaml"]
+CMD ["-config", "/app/config.toml"]