Clarify ecosystem features

#3197 overhauled the client feature table of the ecosystem section of the website. [This](https://github.com/matrix-org/matrix.org/pull/3197#discussion_r2758888830) and other PRs trying to maintain client feature flags keep reminding us that these features are not well explained.

We should try to find clearer definitions of each feature and document the in places such as [templates/macros/clients.html](https://github.com/matrix-org/matrix.org/pull/3197/files/97732f460de191a03e3bd75d208f6941db2977a8#diff-63e40598efc5bf3e9cfc2c820ad3e4493473e1e1dc7ff7fc937d331a2ad8e6d3), CONTENT.md and where ever else relevant (maybe deduplicate and referencing just a single place for sanity?). This includes defining what "supported = true" means in general.

We should also add a feature to the ecosystem pages to learn more about the definition of a feature. I think something like a question mark icon for each feature entry which when hovering or clicking will display the explanation text would work.

We also need to expand the feature system from a boolean yes/no to a more nuanced yes/no/unknown and even consider a "yes, but", e.g. Element X advertises call support, but they are not Matrix compliant at this time; or some features only being available behind a feature flag setting, or partially implemented (e.g. displaying but not sending sticker packs)

Start with templates/macros/clients.html, CONTENT.md, and the discussion on PR #3197 to inventory the current ecosystem feature definitions and boolean assumptions. Define a single clear source of explanations, connect feature entries to those explanations, and specify how unknown, partial, or non-compliant support should be represented. Done means the definitions and the ecosystem presentation consistently cover the requested feature states.

Written by the indexing model from the issue text.