docs: document platform branch procedure #912

Job	Run time
Freeze Code	23s
Rebase platform branches (imx8-fpu-ver)	11s
Rebase platform branches (exynos5-ver)	14s
Proof (AARCH64)	12m 9s
Proof (ARM)	15m 36s
Proof (ARM, 1)	17m 2s
Proof (AARCH64, 1)	12m 6s
Proof (ARM_HYP)	13m 56s
Proof (ARM_HYP, 1)	14m 39s
Proof (RISCV64, 1)	14m 39s
Proof (RISCV64)	14m 17s
Proof (X64)	12m 53s
Proof (X64, 1)	12m 48s
Deploy manifest	1m 20s
	2h 22m 13s

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