Add docs for tracing_subscriber/slog

[nick42d]

Closes #67.

1. Utilises the doc_cfg attribute to show documentation for non-default docs by default on docs.rs. This also benefits the slog feature. https://users.rust-lang.org/t/how-to-document-optional-features-in-api-docs/64577
2. Added some implementation information and information about Adjust the log level of tracing #66 to the pre-existing docs

This will build as expected on docs.rs with no additional effort as they run nightly rust. However, to build docs locally, you will need to run the following command to mimic docs.rs environment.

cargo +nightly rustdoc -p tui-logger --all-features -- --cfg docsrs

Here's an example of the new page for TuiTracingSubscriberLayer