Clarify workflow step functionality

[jrhender]

Workflows currently include step functionality (https://w3c-ccg.github.io/vc-api/#create-workflow). The Create Workflow request body includes a nextStep property which provides a clear property to define a linear sequence of steps. However, the intent of workflow steps is to support more complex workflows such as ones that include branching logic. The spec doesn't clearly explain this intent or how branching logic could be configured.

Note that this was discussed in the 2025-01-07 call. @dlongley mentioned that one way that branching could be configured is using the step templates functionality.

I propose that the following be added to the spec:

• Text that describes that the intent of the steps feature is to support branching and repeated steps.
• Text that describes how templates could be used to implement branching
  • Note that I'm unsure if this text should be normative

[msporny]

The group discussed this on 2025-01-14 and agreed that it was ready for PR.