[S] Change in coreboot[main]: Documentation/getting_started/cbmem.md: Add cbmem documentation

[9elements QA (Code Review)]

9elements QA has posted comments on this change by Nick Kochlowski. ( https://review.coreboot.org/c/coreboot/+/84761?usp=email )

Change subject: Documentation/getting_started/cbmem.md: Add cbmem documentation
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Patch Set 3:

Automatic boot test returned (PASS/FAIL/TOTAL): 11 / 1 / 12

PASS: x86_32  "Hermes CFL" , build config PRODRIVE_HERMES_ 
     and payload TianoCore_UefiPayloadPkg : https://lava.9esec.io/r/238257
PASS: x86_32  "Hermes CFL" , build config PRODRIVE_HERMES 
     and payload TianoCore_UefiPayloadPkg : https://lava.9esec.io/r/238255
PASS: x86_32  "ThinkPad T500" , build config LENOVO_T500 
     and payload SeaBIOS : https://lava.9esec.io/r/238254
PASS: x86_32  "HP Z220 SFF Workstation" , build config HP_Z220_SFF_WORKSTATION 
     and payload LinuxBoot_BB_kexec : https://lava.9esec.io/r/238253
PASS: x86_64  "HP Compaq 8200 Elite SFF PC" , build config HP_COMPAQ_8200_ELITE_SFF_PC.X86_64 
     and payload TianoCore : https://lava.9esec.io/r/238252
PASS: x86_64  "HP Compaq 8200 Elite SFF PC" , build config HP_COMPAQ_8200_ELITE_SFF_PC.X86_64 
     and payload SeaBIOS : https://lava.9esec.io/r/238251
PASS: x86_32  "HP Compaq 8200 Elite SFF PC" , build config HP_COMPAQ_8200_ELITE_SFF_PC 
     and payload TianoCore : https://lava.9esec.io/r/238250
PASS: x86_32  "HP Compaq 8200 Elite SFF PC" , build config HP_COMPAQ_8200_ELITE_SFF_PC 
     and payload SeaBIOS : https://lava.9esec.io/r/238249
PASS: x86_64  "QEMU x86 i440fx/piix4" , build config EMULATION_QEMU_X86_I440FX_X86_64 
     and payload SeaBIOS : https://lava.9esec.io/r/238248
PASS: x86_32  "QEMU x86 i440fx/piix4" , build config EMULATION_QEMU_X86_I440FX_ASAN 
     and payload SeaBIOS : https://lava.9esec.io/r/238247
FAIL: x86_32  "QEMU x86 i440fx/piix4" , build config EMULATION_QEMU_X86_I440FX_ 
     and payload SeaBIOS : https://lava.9esec.io/r/238246
PASS: x86_32  "QEMU x86 i440fx/piix4" , build config EMULATION_QEMU_X86_I440FX 
     and payload SeaBIOS : https://lava.9esec.io/r/238245

Please note: This test is under development and might not be accurate at all!