docs: add tracing-forest to "Related Crates" section (#1935)

## Motivation

This PR adds [`tracing-forest`](https://crates.io/crates/tracing-forest)
to the list of related crates. `tracing-forest` provides contextual
coherence when writing logs, and is being used by projects like
[Kanidm](https://github.com/kanidm/kanidm/).

## Solution

I added it to the documentation in `lib.rs` and in the README. Am I
missing anything else?

README.md

@@ -399,6 +399,8 @@ are not maintained by the `tokio` project. These include:
 - [`tracing-elastic-apm`] provides a layer for reporting traces to [Elastic APM].
 - [`tracing-etw`] provides a layer for emitting Windows [ETW] events.
 - [`sentry-tracing`] provides a layer for reporting events and traces to [Sentry].
+- [`tracing-forest`] provides a subscriber that preserves contextual coherence by 
+  grouping together logs from the same spans during writing.
 
 (if you're the maintainer of a `tracing` ecosystem crate not in this list,
 please let us know!)
@@ -433,6 +435,7 @@ please let us know!)
 [ETW]: https://docs.microsoft.com/en-us/windows/win32/etw/about-event-tracing
 [`sentry-tracing`]: https://crates.io/crates/sentry-tracing
 [Sentry]: https://sentry.io/welcome/
+[`tracing-forest`]: https://crates.io/crates/tracing-forest
 
 **Note:** that some of the ecosystem crates are currently unreleased and
 undergoing active development. They may be less stable than `tracing` and

tracing/src/lib.rs

@@ -743,6 +743,8 @@
 //!  - [`tracing-fluent-assertions`] provides a fluent assertions-style testing
 //!    framework for validating the behavior of `tracing` spans.
 //!  - [`sentry-tracing`] provides a layer for reporting events and traces to [Sentry].
+//!  - [`tracing-forest`] provides a subscriber that preserves contextual coherence by
+//!    grouping together logs from the same spans during writing.
 //!
 //! If you're the maintainer of a `tracing` ecosystem crate not listed above,
 //! please let us know! We'd love to add your project to the list!
@@ -776,6 +778,7 @@
 //! [`tracing-fluent-assertions`]: https://crates.io/crates/tracing-fluent-assertions
 //! [`sentry-tracing`]: https://crates.io/crates/sentry-tracing
 //! [Sentry]: https://sentry.io/welcome/
+//! [`tracing-forest`]: https://crates.io/crates/tracing-forest
 //!
 //! <pre class="ignore" style="white-space:normal;font:inherit;">
 //!     <strong>Note</strong>: Some of these ecosystem crates are currently