docs: summarizing how network upgrades impact Lotus and its key dependencies

[BigLep]

This is an attempt to summarize the relationships between Lotus and its key dependencies that are affected during a network upgrade. Some of this came out of discussing version schemes in filecoin-project/filecoin-ffi#468.

Checklist

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• New features have usage guidelines and / or documentation updates in
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[BigLep]

@Stebalien or @rvagg : @rjan90 and I were speaking and we didn't have full clarity on the filecoin-ffi → go-state-types dependency. I see filecoin-ffi has a go.mod dependency on it, but the update dependency flow that @rjan90 uses doesn't involve making a go-state-type update to filecoin-ffi. Could you please shed some light so we could get this documented?