[DOC] Improve light-client documentation

[volovyks]

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We have a gap in our docs. Light Clients are not well-documented. There is no intro, motivation somebody should use it, or clear instructions on how that can be done.
https://nomicon.io/ChainSpec/LightClient is a deep technical doc. Too low-level for somebody who wants to use it.
https://github.com/near/near-light-client does not give clear instructions on how to start using it.
For reference, Ethereum doc starts from the very beginning: https://ethereum.org/en/developers/docs/nodes-and-clients/light-clients/