Reorganizes and adds new info to the tracing docs. Specifically:
breaks tracing pages into new section --> docs/evm-tracing
adds new landing page
reorganizes built-in tracers info and adds call/return examples to each tracer
adds documentation for diffMode
adds to explanation of state storage and reexec
only minor changes to custom-tracers.md
adds state storage image from Sina's Devcon talk
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