Document module split in coding guidelines

[mx-psi]

On #11216 and others we have taken the approach of splitting existing modules into the parts that we intend to stabilize. This is not documented on our coding guidelines, but it should be!

We want to document the following:

• What are the rough rules of thumb we use for deciding when to split into different modules vs different packages in the same module
• What are some situations where we will always split modules (experimental stuff, testing helpers)
• How to go about splitting a new module
  • Roughly, what changes are necessary
  • How to make sure there are no issues during the release because of the new modules