error-stack: clarify documentation of HookContext (#1987)

## 🌟 What is the purpose of this PR?

#1693 separated `fmt::HookContext` into `hook::HookContext` and `fmt::HookContext`, so that it can be used in #1558. While working, the problem is that documentation doesn't render correctly because `hook::HookContext` is private in public. Therefore unreachable rustdoc is unable to navigate to the type, and the documentation is incomplete.

This PR moves to a macro-based approach to enable better documentation. Instead of using an unreachable public type, we have a macro that implements the necessary common functions. This is by far not ideal, but I found this to be the best way without compromising #1558 or future hooks.