docs: Add warning to changes in board variants in zephyr 4.1

[Ndot]

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.

The docs made no reference to the new board variants in zephyr which will end up in failed builds when using Zephyr 4.1. (#3297)

I added a warning calling attention to it in "Local Toolchain/Building and Flashing" page. Although I'm guessing the project is moving on with the new version of zephyr so probably all the examples should be rewritten to reflect the new way to reference boards.

[petejohanson]

Suggested change

	Since Zephyr 4.1 the argument `-b` needs to be set according to the new board
	variants. E.g. instead of `planck_rev6` it should be `planck//zmk`. Read more about
	Since moving to Zephyr 4.1, the board IDs for boards in ZMK/Zephyr need include the ZMK board
	variants. E.g. instead of `planck_rev6` it should be `planck//zmk`. Read more about

or something like this.

And we should probably fix the -b proton_c a few lines up, while we're at it.