Closes #16.

The Instructions section of the README.md file for the Arduino Cellular library has been updated to provide clearer and more detailed setup instructions, with improved visuals, including:

• 📐 SIM card form factor: Specifically state SIM card size (nano)
• 👆 SIM slot location: Made clear that the nano SIM card is inserted into the Portenta Mid Carrier. A photograph from above is also provided that better shows the size and location
• 🛠️ Fixing PRO 4G Module: Added step explaining how to physically fix the PRO 4G Module to prevent damage
• 📻 Antenna Connections: Explicitly mentioned which antenna should be connected to which microUFL connector for both the cellular and GNSS antenna. Reference is made to the Arduino Pro 4G Module Antennas Kit since the antenna is NOT included by default with the PRO 4G GNSS/EMEA modules
• 🔌 Power supply: Use of external power supply (not USB) is mentioned, based on approach borrowed from @marqdevx

Thanks to @marqdevx and @i-herrera for their feedback, and @kengdahl for taking the images.

@jcarolinares these instructions could be useful for content development