Prefixes not directly connected to databases

[rartino]

During the OPTIMADE call came up the question that we already have two types of prefixes in https://providers.optimade.org/providers.json

Those that are specifically tied to databases, and those that are more "general", i.e., tied to a tool, or, we discussed things like _cif_ that refers to a definitions out of a different standard.

We seemingly agreed that it is in our interest to keep allowing both these types, and that requires at least a bit of clarification in the specification as it is presently written. But also the consensus seemed to be that we still want to keep them in the same register since they occupy the same namespace (i.e., https://providers.optimade.org/providers.json ). There was also the point that a prefix not tied to a database provider still should have a maintainer pointed out (so, presumably we point out that entity in the prefix database).

This issue is for doing the necessary update to the text.

There may be a good idea to also separately discuss the idea of a _cif_ prefix. (Actually lifted the first time in #143 but rejected there.)