The arguments to `circup example` are mysterious

The arguments to `circup example` are not at all obvious, and the help message doesn't help too much.

I finally figured out it was `circup example ble/ble_uart_echo_test`. This is not a real pathname, despite the `/`.

It is not:
```
circup example adafruit_ble/examples/ble_uart_echo_test
circup example adafruit_ble/examples/ble_uart_echo_test.py
circup example adafruit_ble/ble_uart_echo_test
circup example adafruit_ble/ble_uart_echo_test.py
circup example ble/ble_uart_echo_test.py
circup example ble ble_uart_echo_test
```
I found `circup example --list` and then discovered I could do `circup example ble --list`, but some examples in the help text would be helpful.

Start by running `circup example --help`, `circup example --list`, and `circup example ble --list` to understand the current CLI help and valid argument format. Update the help text with representative working examples, then rerun the help commands to confirm the examples are clear and accurate.

Written by the indexing model from the issue text.