* Refresh & improve leader documentation page
- register_code/unregister_code are not the recommanded way to do macro.
- Provide some details I wish I had found when first used the leader
functionality.
* Add old way to use macro.
* Fix dead link to USB keycodes doc
Link was dead and the fresher version I could find on usb.org is still older than this one.
Thus, WaybackMachine seems the best option.
* Fix dead link to USB keycodes doc, with 2 options
Give the WaybackMachine link (fresher and for reference of the content of the original link) and the usb.org one (older)