Browsers may not want to load files outside the folder, but they may be
okay with these relative references within the folder.

- Also partial cleanup of Marlin_main.cpp with a smidgen of Doxygen.

Hopefully this is helpful to organization and future expansion….

Hopefully this is helpful to organization and future expansion….

Hopefully this is helpful to organization and future expansion….

## Add: Board for Vellemann K8200 (derived from 3Drag)

* Boards.h: updated comment for board 3Drag - K8200 removed
* Boards.h: added `BOARD_K8200` with ID 78

* language.h.: machine name and firmware URL for new board

* pins.h: added `K8200` to any `#if` with `3Drag` and corresponding
comments

## Update Boards.h for Vellemann K8200 - comment only

* Boards.h: updated comment for board 3Drag - K8200 uses same board

Added boards.h which contains defines for all the board numbers, plus
some shorthand macros for less typing.

Added boards.h which contains defines for all the board numbers, plus
some shorthand macros for less typing.