Deprecate matrix.to for FediLinks

Nobody has claimed this yet.

[The Fedi Links project](https://fedilinks.org/) is a project to introduce URI fallbacks in spite of OS/browser/client support. The main specification for the fallback behaviour is https://fedilinks.org/2 .

It would be neat to deprecate matrix.to entirely, tho we recognize that's a few years off. A good start would be to decide on what a hypothetical web+matrix URI should look like. Ideally, a web+matrix URI fallback would support viewing public rooms as a guest if the server allows for it, directly on the home server. This would help bridge the world of matrix with the world of the web.

Plus, what happens if the project dies/runs out of funding/gets bought out? A native fallback cannot be controlled, while matrix.to can be killed. Deprecating matrix.to would help with the [bus factor](https://en.wikipedia.org/wiki/Bus_factor).

Start by reading the Fedi Links specification at https://fedilinks.org/2 and the issue's discussion of a hypothetical web+matrix URI. Define a proposed URI shape, how it would support guest viewing of public rooms on the home server, and what deprecating matrix.to would require; done means the proposal is documented and agreed upon.

Written by the indexing model from the issue text.