[Keymap] Mekberg kbd6x keymap (#7061)

[Keymap] Mekberg kbd6x keymap (#7061)

* First draft of my layout

* Improved layout and cleanup of files

* Update keymap and add rules

* Add keymap.h with permissive_hold setting

* Rename keymap.h to correct name config.h

* Add next/prev and special lock key to Fn layer

* Use correct modifier in MY_LOCK command

* Removed unnecessary filler defines

* Add build instructions to README

* Move RGB controls to more logical up/down key positions, move next/prev controls, remove del from Fn layer

* Fix wrong placeholders and fix up formatting

* Remove unused code

* Clarify comments on custom defines

* Update keyboards/kbdfans/kbd6x/keymaps/mekberg/rules.mk

Co-Authored-By: fauxpark <fauxpark@gmail.com>