add Q# package registry document (#1932)

A basic document that serves the purpose of a package registry, listing
all packages we are aware of.

Most information about the package should be on the package page itself,
so here we can just list titles, summaries, and links.

---------

Co-authored-by: Stefan J. Wernli <[email protected]>
Co-authored-by: César Zaragoza Cortés <[email protected]>

library/Registry.md

@@ -0,0 +1,22 @@
+# Q# Library Registry
+
+This document is a list of notable Q# library projects. If you have a library that you would like to add to this list, please open a pull request. If you have a Q# project that you'd like to share as a library, please see the [documentation on how to do this.](https://learn.microsoft.com/en-us/azure/quantum/how-to-work-with-qsharp-projects?tabs=tabid-qsharp%2Ctabid-qsharp-run#configuring-q-projects-as-external-dependencies).
+
+## Unstable
+
+[link](https://github.com/microsoft/qsharp/tree/main/library/unstable)
+
+A general staging ground for some useful APIs that have not yet been stabilized into the standard library.
+
+## Signed
+
+[link](https://github.com/microsoft/qsharp/tree/main/library/signed)
+
+Defines types and functions to work with signed qubit-based integers.
+
+## FixedPoint
+
+[link](https://github.com/microsoft/qsharp/tree/main/library/fixed_point)
+
+Types and functions for fixed-point arithmetic with qubits.
+