Add a section for products built on top of Matrix

[thibaultamartin]

We need a new section for products that are (proudly!) built on top of Matrix but are not vanilla Matrix (as opposed to the rest of hosting providers).

Triggered by #2206
Related to #1542

[HarHarLinks]

We also need to find guidelines for what constitutes a "hosting provider".

One idea: if a product also want to be listed as hosting provider because it technically is just (backwards) compatible to "plain" matrix, then it needs to have a section or page on the website to link to that clearly states that, so someone looking for hosting doesn't get confused with the product.

[gnunicorn]

not vanilla Matrix

Well, that would require some definition of "vanilla Matrix" then, I suppose ;) .

As of now Acter is working with any default homeserver just fine, the super invites are completely optional (as are other features in other clients: only available if the homeserver supports them). All additional things it offers are just custom messages within the default Matrix system not yet fully specc'ed (as other clients, like element, also do it) but not interfering with any server-side stuff. Which can just be ignored by any other client and chat would still be fully compatible between them either way. it doesn't do threads, but neither do some other clients... but it does images and video, which some CLI clients do not even support ... so ... what's Vanilla Matrix again ;) ?