Refresh our relationships page

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Our relationships page is structured via the headings "conventions" and "manual configuration"; the modeling docs used to follow this, but we've shifted away to more user-centric structures based on what features and what people are looking for. We may want to do the same here, i.e. have top-level headings for one-to-one, one-to-many, many-to-many, and structure the convention vs. configuration information under those.

We could also consider splitting this into multiple sub-pages, as the page is quite long, and topics such as many-to-many could definitely benefit from a dedicated page.