Deprecate LAYOUT() macro in favor of JSON matrix definitions

* Indicate <keyboard>.h LAYOUT() deprecation in understanding_qmk.md

Initial documentation uses <keyboard>.json to define matrix pin definitions, rather than having users #define the LAYOUT() macro in <keyboard>.h  -  This change brings this docs inline with the [porting_guidelines](https://docs.qmk.fm/porting_your_keyboard_to_qmk),  QMK MSYS will also throw an error if this is attempted.

* Update understanding_qmk.md

enclosed '<' using backtick

* style guideline, now builds correctly

prior version indicates directory, new indicates config. still the same idea.

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