light client guide: concepts/vocabulary

[colin-axner]

Summary

Add documentation for the concepts and vocabulary for creating a light client. Should explain all the defined interfaces. Explanation of each of the functions within the interface will occur in followup documentation

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[chatton]

The link in the description was pointing at a feature branch. I believe this is where we should look for the interfaces now.

[damiannolan]

I can take a stab at getting started on this. This should be included in "Overview" (page 1), right? And lay out some of the basics with descriptions of what each interface is used for.