docs: add guide chapter detailing current way to share classes between modules

guide/src/SUMMARY.md

@@ -32,6 +32,7 @@
 - [Performance](performance.md)
 - [Type stub generation and introspection](type-stub.md)
 - [Advanced topics](advanced.md)
+  - [Sharing types between packages](advanced/sharing-types.md)
 - [Building and distribution](building-and-distribution.md)
   - [Supporting multiple Python versions](building-and-distribution/multiple-python-versions.md)
 - [Useful crates](ecosystem.md)

guide/src/advanced.md

@@ -6,3 +6,9 @@ PyO3 exposes much of Python's C API through the `ffi` module.
 
 The C API is naturally unsafe and requires you to manage reference counts, errors and specific invariants yourself.
 Please refer to the [C API Reference Manual](https://docs.python.org/3/c-api/) and [The Rustonomicon](https://doc.rust-lang.org/nightly/nomicon/ffi.html) before using any function from that API.
+
+## Sharing types between multiple PyO3 modules
+
+It is possible (but complicated) to share types between multiple PyO3 packages which can be compiled and installed separately.
+This allows for ecosystems of functionality to be built similar to the many packages built on top of NumPy.
+While PyO3 does not yet have any built-in support for doing this, the next sub-chapter of this guide describes the general approach to doing this, as well as the safety limitations to be aware of.