docs: clean up tracing_subscriber::fmt formatter docs

[hawkw]

Motivation

Currently, the documentation for the fmt module shows examples of the
output for each formatter in the top-level module. This is a lot of text
that makes finding other things in the documentation more difficult.
Meanwhile, the type-level docs for the various formatters are quite
terse and don't offer much useful information.

Solution

This branch moves the example output to the type-level docs for the
various formatters, and links to it from the list of formatters. I've
also updated and expanded some of the documentation for the formatters,
and added a similar list of formatters to the fmt::format module.
Hopefully, this makes things easier to navigate?

Finally, I've re-generated all example output with the latest versions
of each formatter.

Alternatives

Alternatively, we could not move the example output. The initial
intention for putting it in the top-level fmt module docs was to make
it very easy to find what the different formats look like. Perhaps
this is worth having a lot of text to scroll through in the
module-level docs?

[hawkw]

cc @davidbarsky @lilyball I would love your opinions on the docs changes here!

[davidbarsky]

Yeah, I prefer this layout.

[lilyball]

I agree, this new layout is better. If I want to compare formats I can just pop open the pages in tabs, which actually makes it easier to flip between the tabs to compare. It also means I can just jump directly to the format explanation by searching for the type without having to walk back up to the tracing_subscriber::fmt module to find them.