feat: add `Doc::import_file` and `Doc::export_file` #5833

Job	Run time
cargo deny	34s
Checking fmt and docs	1m 20s
Build and test (Nix) (ubuntu-latest, nightly)	4m 44s
Build and test (Windows) (windows-latest, nightly, x86_64-pc-windows-msvc)	4m 30s
Build and test (Nix) (ubuntu-latest, stable)	4m 43s
Build and test (Windows) (windows-latest, stable, x86_64-pc-windows-msvc)	4m 31s
Build and test (Nix) (macOS-arm-latest, nightly)	4m 41s
Build and test (Nix) (macOS-arm-latest, stable)	4m 42s
clippy_check	2m 51s
Cross compile (i686-unknown-linux-gnu)	3m 11s
Minimal Supported Rust Version	1m 48s
Cross compile (armv7-linux-androideabi)	36s
Cross compile (aarch64-linux-android)	36s
Run network simulations/benchmarks	3m 21s
Build release binaries	1s
	42m 9s

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