docs: add ADR for pathway catalog and content split

[Agrendalath]

This adds an ADR describing a separation of concerns between the marketing and authoring parts of a pathway.

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]

Can you also please add something about the dependency relationship, i.e. openedx_content knows about openedx_catalog, but not vice-versa? That has some implications elsewhere (e.g. Items are allowed to directly reference CourseRuns, and the mapping of CatalogPathway to Pathway content has to be in openedx_content).

[ormsbee]

From a code point of view, I think we should always force it to be specified, and we can create a default database entry for "Pathway". If we do it with a hardcoded fallback-in-code, we're more likely to get inconsistent behaviors.