Document criteria for "server" ecosystem project type

As a part of a wider effort, let's define what we are referring to when we use the term "server" 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 by reading the living criteria in the issue and reviewing the ecosystem project pages on matrix.org to understand how “server” is currently used. No file or test is named, so first locate the page or content source that defines ecosystem project types. Done means the agreed server criteria are documented there and the wording is clear enough to apply consistently.

Written by the indexing model from the issue text.