docs: add unsafe code policy #100

Job	Run time
Cargo check	26s
Machete dependencies checks	1m 57s
Build with each feature combination	22s
Code formatting checks	17s
Minimal dependency versions build	24s
Clippy lint checks	20s
Build & test (stable, windows-latest)	1m 22s
Build & test (1.88, windows-latest)	1m 4s
Build & test (nightly, windows-latest)	55s
Build & test (stable, ubuntu-latest)	23s
Build & test (1.88, macos-latest)	32s
Build & test (1.88, ubuntu-latest)	25s
Build & test (nightly, macos-latest)	47s
Build & test (nightly, ubuntu-latest)	27s
Build & test (stable, macos-latest)	51s
Miri test	1m 4s
	11m 36s

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