Develop guidelines and criteria for inclusion in the circuit library

[kevinsung]

The circuit library has evolved significantly over the course of Qiskit's history. Various things have been added and then removed, and its current state does not necessarily reflect any clear vision on what is supposed to be included, or not. The goal of this issue is to develop clear guidelines and criteria for inclusion. Developing these guidelines and criteria will enable us to evaluate whether existing entries in the library can be removed and whether new proposals should be accepted.

We can take as a starting point the opening paragraph of the circuit library's documentation:

The circuit library is a collection of well-studied and valuable circuits, directives, and gates. We call them valuable for different reasons, for instance they can serve as building blocks for algorithms or they are circuits that we think are hard to simulate classically.

Also, we can take #13573 as a test case.