Add a section about debugging reactivity/subscriptions

The reactivity guide or a section in the reference should document how to debug dependencies for reactive scopes like components and memos. [Vue](https://vuejs.org/guide/extras/reactivity-in-depth.html#reactivity-debugging) has a section we could draw inspiration from

Start with the reactivity guide or the relevant reference section and review how reactive scopes such as components and memos expose dependency information. Use Vue’s reactivity-debugging section as inspiration, and consider the work complete when the documentation explains how to debug those dependencies.

Written by the indexing model from the issue text.