docs: add ADR for mapping pathway items to fulfillment

[Agrendalath]

This adds an ADR describing how the Pathway Items can be fulfilled.

Related PRs:

• docs: add ADR for pathway catalog and content split #761
• docs: add ADR for pathway and pathway item boundaries #762
• docs: add ADR for mapping pathway items to fulfillment #763
• docs: add ADR for pathway credentials #764

Private-ref: BB-10965

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[ormsbee]

Reading this does make me think of a weird edge case that will probably require an async task, which is:

What happens when the following sequence of events happens:

1. A learner successfully completes Course Run A, which does not count for anything in the Pathway.
2. The learner enrolls in the Pathway.
3. The author then edits a Pathway Item so that Course Run A does fulfill it.

This slips by both our typical Item satisfaction checks:

• The one-time check on Pathway enrollment that goes though everything the student has already done.
• The on-passing event check that we can catch when they pass the course run, which we'd then check against all active pathway item fulfillment criteria.

So the only thing I can think of to address this automatically would be to fire off an async task that goes through all the students currently enrolled in that Pathway whenever changes to a Pathway Item are published, and give retroactive Pathway Item credit for anything new that was added to that Item.

[Agrendalath]

@ormsbee, @bradenmacdonald, I described evaluation scenarios in a1ee30d.

[ormsbee]

I plan to merge this tomorrow afternoon (1 week after it was initially opened), unless anyone has objections.