Refactor non-normative workflow explanation to "exchange" explanation.

[msporny]

This PR is based on ACTIONS requested to be taken on the last two VC API calls (see summary at top of transcription): https://w3c-ccg.github.io/meetings/2022-02-01-vcapi/ and the latest here https://w3c-ccg.github.io/meetings/2022-02-08-vcapi/

The goals here are to:

• Refactor the existing traceability holder APIs (in a backwards-compatible way) using input from workflows section and 'exchanges' discussion on VC API call [DONE].
• Merge non-normative text from workflows into new refactored sections (with appropriate wording changes, e.g., "workflow" -> "exchange").
• Remove workflow sections once everything has been merged together.

This PR achieves the last two items. These are all non-normative changes, you can see all the modified content here:

https://pr-preview.s3.amazonaws.com/w3c-ccg/vc-api/pull/261.html#exchange-examples

The target merge window is in 7 days, after the VC API call next week.

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Preview | Diff

[mavarley]

This PR captures a lot of the core concepts for exchanges and is a good baseline to build up from.
Specifically, this PR allows further specification on issue #259 and other exchange API discovery and specification topics.