Document criteria for "distribution" ecosystem project type

As a part of a wider effort, let's define what we are referring to when we use the term "distribution" within the context of the ecosystem project pages.

> [!IMPORTANT]
> The criteria documented here are very much a work in progress. If you’re looking to provide feedback, please remember that this comment is a living document. Quote the specific section you want to discuss, as it may be edited or removed.

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Start with the ecosystem project pages and the living criteria in this issue to understand how “distribution” is currently used. Document the agreed criteria for this project type, keeping the sections easy to review and confirming the result with the issue discussion.

Written by the indexing model from the issue text.