doc: Recommend tracking Cargo.lock with rust-toolchain files

[MoSal]

Current use-case where not doing this causes issues:

cargo new ttt
cd ttt
sed -i '/edition/s;2021;2018;' Cargo.toml
echo 'criterion = "0.3"' >> Cargo.toml
echo '1.55' > rust-toolchain
cargo check

Issue triggered by an indirect dependency requiring a newer edition than what the pinned toolchain supports.