Update getting started docs to reflect gdb python requirements

[ghost]

Describe the bug
west debug fails with error

/home/user/zephyr-sdk-0.12.4/arm-zephyr-eabi/bin/arm-zephyr-eabi-gdb: error while loading shared libraries: libpython3.8.so.1.0: cannot open shared object file: No such file or directory
FATAL ERROR: command exited with status 127: /home/user/zephyr-sdk-0.12.4/arm-zephyr-eabi/bin/arm-zephyr-eabi-gdb /home/user/app/build/zephyr/zephyr.elf -ex 'target remote :2331' -ex 'monitor halt' -ex 'monitor reset' -ex load

I followed the getting started guide and the application development steps to create a very simple hello world program outside of the zephyr base directory. It compiles and flashes fine however when trying to get debug functioning I get the above error. I am using a jlink and mimxrt1060_evk.

Based on some stack exchange posts of other people missing that library I installed libpython3.8-dev and that solved the problem. Perhaps this should be added to the list of dependencies? Or did I miss something in the documentation?

To Reproduce
Steps to reproduce the behavior:

1. west build
2. west flash
3. west debug

Expected behavior
GDB starts with no errors

Environment (please complete the following information):

• OS: Linux Ubuntu 18.04
• Toolchain: Zephyr SDK, West

[galak]

You need to do:
sudo apt get install python3.8-dev

[galak]

This is a known requirement. You can also utilize the /home/user/zephyr-sdk-0.12.4/arm-zephyr-eabi/bin/arm-zephyr-eabi-gdb-no-py version

[jfischer-no]

Unfortunately, exactly this Python version is no longer available on some distributions.

[ghost]

You need to do:
sudo apt get install python3.8-dev

If specifically python3.8 is required the getting started documentation should be changed to reflect that. Currently it just calls out installing python3 which on 18.04 installs python3.6.9

I appreciate that your solution solves my problem, and I had even already found a solution before I posted. The issue here is more with the documentation.