contrib/sni-router: render mtg-config.toml from a tracked .example
Track `mtg-config.toml.example` with `secret = "${MTG_SECRET}"`; the
rendered `mtg-config.toml` and local `.env` are gitignored, so the
secret never lands in a tracked file.
Quick start switches from "paste the secret into mtg-config.toml" to
either `envsubst < mtg-config.toml.example > mtg-config.toml` or
`cp` + hand-edit `${MTG_SECRET}` for users without envsubst.
After #502 made DOMAIN env-driven, the secret was the last hand-edit
of a tracked file in the example. Follow-up to #506.
Alexey Dolotov