refactor(behaviors): Minor RAM usage tweaks

[petejohanson]

Follow up cleanup of some config structs not marked as const.

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.

[Nick-Munnich]

Couple more points in the example code in the docs which could use a const added, along with a note under the corresponding section in the docs that _config should always be const.

[petejohanson]

Couple more points in the example code in the docs which could use a const added, along with a note under the corresponding section in the docs that _config should always be const.

Added the note, but I'm not seeing any other declared instances of the config data in the new behavior guide that would get updated.

[Nick-Munnich]

You're right, mb, I misread a bit. LGTM