To add The Adafruit CLUE as a board in Arduino IDE, go to File → Preferences, and add an additional board manager URL for https://www.adafruit.com/package_adafruit_index.json. Then if you go to Tools → Board → Board Manager, you can install Adafruit nRF52 which is the one that will add CLUE support. You can also install Adafruit AVR and Adafruit SAMD for other boards. When you switch the board to the Adafruit CLUE, go to File → Examples, and at the bottom of the list you should see many examples specific to the Adafruit CLUE.
I'm not positive if this step is necessary yet, installing adafruit-nrfutil.
sudo apt install python3-pip # Debian python3 -m pip install adafruit-nrfutil adafruit-nrfutil version