docs: Add a per-module log level config example

[wlinna]

This closes #219

I tested that it compiles and runs with...

cargo run --example per_module_log_levels -F starter-log

... but since the only logger here is the example itself, you wouldn't know it actually works just by running the example

[tisonkun]

Thanks for your contribution @wlinna!

but since the only logger here is the example itself, you wouldn't know it actually works just by running the example

I think it's OK for now to demonstrate the usage. We may improve it later when finding better expression.