Some revisions to cbbrowne Planck keymap, and a preliminary xd75 keymap (#1715)

Some revisions to cbbrowne Planck keymap, and a preliminary xd75 keymap (#1715)

* Add HOME/END keys as upper/lower on arrow-up/down

* Reduce .hex file size by turning off unneeded options

* Put digit keypad onto left hand upon RAISE; this will sometimes be preferable to double-hits of right hand

* Latest super latest version merge

* cbbrowne keymap for XD75re

* starting notes on XD75re keymap plans

* First draft of bottom row of QWERTY

* Switch my special bottom line over to QCENT

* Dunno

* Filling in wanted keys, bit by bit...

* Add copyright, extra macro

* Clean up comments, remove some experimental code I didn't like

* TODO plans for xd75re

* clean up keyboard layout

* QCENT2 is my new experiment for the main keyboard...

* Add a few more main layer keys, and modify LOWER to shift things outwards to conform with main layer

* Clean up RAISE layer to conform with main layer, remove QCENT layer as QCENT2 is the new thing

* More xd75 changes, now that I actually have it in hand

* shift keymap around, as original attempt was a bit too aggressive in keeping to the edges

* more revs to XD75

* Dropping parts of the centre keypad in favor of Keys I Really Need

* Improve documentation to conform with how builds are done now

* Improve documentation to conform with how builds are done now

* Add cbbrowne rules file as alternative to having the rules in Makefile

* Makefile not needed anymore for individual keymap