chore: Add a deprecation warning to config/boards

[Nick-Munnich]

Thought it was about time for at least a warning to be spat out. My suggestion would be that after some more time we make using config/boards spit out an error that can be lifted with an extra cmake argument, but I think we should have a warning message first.

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.

[caksoylar]

I think it'd be better to be more explicit here, someone who doesn't follow ZMK might not even know what a "module" is, given this warning will be emitted by GH Actions where config repo == module. Is there a docs page we can link to?

[Nick-Munnich]

With #2456 merged I have now updated this with a link to the appropriate docs page.

[caksoylar]

I think the new shield guide might be more straight-forward to link to, given the module creation page might be a bit overwhelming for people who just want to use a custom shield. Maybe we can add both, with new shield first, what do you think?

[Nick-Munnich]

Good idea, made that change.