refactor(shields): tidbit

[lesshonor]

• Add missing wakeup-source attribute to kscan node.
• Remove partially-functional "tidbit_19key" shield and related documentation in favor of an alternate matrix-transform.
  • See #1132. Given the current flawed status, the near-total absence of functional public usage and the fact that there is no difference between the shields other than their initial/default keymaps, I see no value in preserving _19key.
• Remove Kconfig files under boards.
  • proton_c.conf is blank. nice_nano.conf force-enables RGB underglow in a way that is opaque to end-users and appears to be impossible to override without manually passing cmake args (e.g. -DEXTRA_CONF_FILE=/path/to/disable_underglow.conf or -DCONFIG_ZMK_RGB_UNDERGLOW=n).
• Related cleanups/additions to the README, default .conf files and default .keymap.