Missing examples for reusable services

### Description of defect

Previously this repository contained services that one could easily look at to see how they could be reused in your own code (like the LED service).

Now all the examples seem to have code in which the application is tied up with the service itself, so I can't figure out how standalone resuable services are supposed to look anymore.

I'd like to see an example in which BLE is just one aspect of the program, but doesn't dictate the application itself.

I went from the blinky examples to a BLE enabled app by comparing blinky with those previous examples by comparing them.

Maybe the problem is just that I don't know how to migrate from something that looked like the old code that didn't use the new ble helper library to the new ones that do.

#### What version of the example and mbed-os are you using (tag or sha) ?
6.x

Compare the current blinky and BLE examples with the previously available standalone LED service examples to understand the intended reusable-service structure. Add a BLE example where BLE is one part of a broader application rather than the application itself, and verify that it demonstrates the migration from the older helper-free style.

Written by the indexing model from the issue text.