New reference doc for Custom RPC V2 (#4654)

Thanks for @xlc for the original seed info, I've just fixed it up a bit
and added example links.

I've moved the comparison between eth-rpc-api and frontier outside, as
it is opinionation. I think the content there was good but should live
in the README of the corresponding repos. No strong opinion, happy
either way.

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Co-authored-by: Bryan Chen <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Gonçalo Pestana <[email protected]>
Co-authored-by: command-bot <>