Fixes#7098. The light client documentation moved to the spec repository.
I was not able to figure out what happened to light-client-protocol.md, it was removed in #5252 but no corresponding file exists in the spec repository. Since the spec also discusses the protocol, this change simply links to the spec and removes the non-functional reference.
Alternatively we could link to the top-level [light client doc](https://docs.tendermint.com/master/tendermint-core/light-client.html) if you think that's better.
(cherry picked from commit 48295955ed)
Co-authored-by: M. J. Fromberger <fromberger@interchain.io>
