Better document the extension API

[vitaut]

Improve the docs: https://fmt.dev/latest/api.html#formatting-user-defined-types, particularly documenting what the inputs and the outputs are in parse and format and provide a more interesting example.

[peter-b]

Am I right to suspect the "more interesting example" include suppressing a leading - when a floating point number happens to be rounded to all-zeroes?

[vitaut]

That's not what I had in mind but now I'm considering - suppression as an example =).