Documentation for 0.18.0 failed to build on docs.rs

[romac]

See https://docs.rs/crate/tendermint-light-client/0.18.0

The issue is that I forgot to enable the doc_cfg feature when the docsrs config attribute is enabled.

This feature allows us to show a little badge next to items which are only conditionally defined based on some feature flag, like the sled-based light store.