Improve "Porting" guidance for tap developers

[MeltyBot]

Migrated from GitLab: https://gitlab.com/meltano/sdk/-/issues/169

Originally created by @aaronsteers on 2021-07-12 16:17:15

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The porting guide is not very good. Specifically, it does not describe the exact contract that the SDK expects, which methods or overrides must be overriden, how to migrate State-management code, etc.

For reference, here is the current doc: https://sdk.meltano.com/en/latest/porting.html

We should expand the available docs and collect feedback from one or more developers who have been through the process.

[MeltyBot]

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