[Feat] docs/howto: Add howto-testing-portenta-x8.md

[gmacario]

Signed-off-by: Macario Gianpaolo gmacario@gmail.com

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📑 Description

First experiences with Arduino Portenta X8

✅ Checks

• [x] My pull request adheres to the code style of this project
• [ ] My code requires changes to the documentation
• [x] I have updated the documentation as required
• [x] All the tests have passed

ℹ Additional Information