Document criteria for "SDK" ecosystem project type

As a part of a wider effort, let's define what we are referring to when we use the term "SDK" 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 document described in the issue. Review the current SDK definition and related ecosystem project guidance, then document agreed criteria; the work is done when the meaning of “SDK” is clear and consistently applicable to those pages.

Written by the indexing model from the issue text.