docs: add ADR for pathway and pathway item boundaries

[Agrendalath]

This adds an ADR defining the boundary between a Pathway and its Items.

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]

Minor comment just to get clarification, but no change required.

[ormsbee]

The fact that we know we're going to have to later accommodate much more sophisticated display rules for Pathways makes me think that the Pathway Item ordering is something that's going to be modeled at arm's length, i.e. a M:M simple learner display ordering model, rather than making ordering an intrinsic part of the PathwayItem. With the idea that eventually other more sophisticated M:M models will be added to encode fancier dependency/ordering/grouping rules. Does that sound right to you?

[Agrendalath]

@ormsbee, my idea for the ordered list in the MVP was to allow authors to specify the order of item display (e.g., either the suggested order of completion for learners or the content author's pure preference).
I agree that M:M relations will be reasonable for content groups and prerequisites. However, I'm wondering whether we should actually mix the display order with the prerequisites. If not, perhaps keeping the order directly as a PathwayItem field would be a better choice?

[ormsbee]

I think we have the same concern—I was just thinking both orderings would be M:M relations. Thinking on it more, I'm fine with putting author-ordering on the PathwayItem directly for student display purposes as you describe.

[ormsbee]

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