Add documentation for micrometer-tracing

[mhalbritter]

The auto-configurations have been in main since 3860eb2. We should add some documentation describing what it does and what to do to get tracing running with Zipkin and Wavefront. This comment contains the dependency combinations.

There's some documentation from the Micrometer team, too.

[wilkinsona]

We should add something to the release notes too.

[mhalbritter]

Added a section to the release notes.

[mhalbritter]

I'm working on that one here: https://github.com/mhalbritter/spring-boot/tree/mh/30658-add-documentation-for-micrometer-tracing