Add documention for 09-localhost

[damiannolan]

Summary

Add documentation explaining how the 09-localhost client operates and the usage of the sentinel localhost connection end stored in state.

Docs should clearly state:

• The ClientState structure and its fields.
• VerifyMembership and VerifyNonMembership prove key-value pairs using the core IBC store.
• Client updates on BeginBlock to track LatestHeight of localhost client.
• Usage of sentinel localhost connection end.
• Wiring of AppModuleBasic in app.go, used for registration of types using the interface registry.

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