Add example using GPIO DynPin

[9names]

#585 changed how GPIO worked, and the changelog hinted that it made it easier to use Pins in arrays but did not include any examples, doc-examples, or code snippets that tell you how to do it.
This example should at least provide some hints to new embedded rustaceans on how to use this feature.

[jannic]

This needs an entry in Cargo.toml:

[[example]]
name = "gpio_dyn_pin_array"
required-features = ["critical-section-impl"]