add rtic serial interrupt example

[elwerene]

I'm sorry I didn't get the gdb runnter to run, so I could only test a version with probe-rs (sth similiar like here: https://github.com/elwerene/nrf52833-rtic-serial-interrupt).

It took me some hours to get a working version with lots of help from people hanging around at the nRF Rust Matrix channel.

I also only could test the code on nrf52833. Should I remove the other features, or will it work on them?

[jonas-schievink]

Note that we're not currently merging any more examples due to #358

[elwerene]

Oops.. well that's reasonable! Maybe it's enough to have it as an open pull request so people can find it :)

[elwerene]

I'm feeling stupid now as that's exactly where I could have found a more or less working example in #102 :/

[elwerene]

Could I somehow work this into the documentation? I really don't want anyone else to have to follow my path to UART interrupts.