docs: Project structure decision guide (expand existing best-practices page)

[Holmus]

Gap

The existing Structuring Your Projects page covers when to separate vs share projects, but does not provide a single decision-tree framing based on customer topology. The decision is the most-asked structural question during evaluation and onboarding, and the current page reads as "considerations" rather than "decide like this."

Proposed scope

Add a decision-tree section keyed on customer topology:

Customer topology	Recommendation
Single business product, multiple surfaces (web + mobile + backend)	Single project — shared source of truth for the feature list
Multiple distinct business products under one organisation	One project per business product
Independent service team ownership with no shared product context	Split by ownership; easy to refactor later via flag export
Default when in doubt	Bias toward fewer projects — splitting is reversible, merging is harder

This complements the existing "when to separate vs share" sections rather than replacing them, and lands the default ("bias toward fewer") at the top of the page where it gets read first.