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?