Create some page in docs about possible build flags

[frostmorn]

Currently added only by me are:

USE_EXT_SPI_FOR_SD maps sdcard on spi1, and uses extension header pins :
SPI2_SCK 12
SPI2_MISO 13
SPI2_MOSI 14
SPI2_DEV1_CS 21

FMANAGER_DEBUG enables logging for FileManagerApp

also already exist

NESAPP_INTERLACED to render half of frame in NesApp, odd/even rows of pixels so half per frame.
render full frame(default)

KEIRA_SCREENSHOT_BMP to make screenshots in BMP format. PNG(default)

LILKA_BREADBOARD reduces display spi speed to 40 mHz, 80mHz (default)

KEIRA_RAM_ICON should show ram usage on panel in an icon format instead of numbers(default)

Probably some others, I've zero idea about, anyways, cause their number rise we should create doc for them.

Also worth mentioning some other ones like ARDUINO_USB_CDC_ON_BOOT, CORE_DEBUG_LEVEL etc.

LILKA_BLE after merging #199
would allow to control BLE feature. Note: Doom stucks if BLE enabled now