[PXCT-536] Nano Matter - Remove custom board url

[Hannes7eicher]

What This PR Changes

This PR removes any mentions of the custom board url since the Nano Matter core is now available by default in the Arduino IDE.

Suggestion: Add a getting started guide to maintain all information regarding board installations and first time use in one single place. This way updates don't need to be done to all articles but only to one instead. cc @mcmchris @jcarolinares

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[mcmchris]

The User Manual needs the same changes you applied in other tutorials.

Others look good to me.

[jcarolinares]

Thanks @Hannes7eicher

We follow another approach regarding content for this product where the User Manual is the central part of the experience, being additional tutorials linked to it as an expansion as that document.

User experience has a priority over efficiency. If we improve the user experience by repeating things, we will do and we will change things in all the places is needed.

However, the experience you are referring to is contained on the "First use" section of the User Manual and therefore you can always refer to that sections in other tutorials and part of the documentation if you feel to: https://docs.arduino.cc/tutorials/nano-matter/user-manual/#install-board-core-and-libraries

thank you both, let's keep going!!! :)

[mcmchris]

LGTM now! Thanks @Hannes7eicher

As a personal note: the changes of this task didn't need a new image or further explanation.