docs: add manifest-path config for src layout

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[MukundaKatta]

+1 to documenting this. I used manifest-path yesterday for a workspace
where the bindings crate sits under crates/foo-py/ and the workspace root
is the maturin-managed Python project. Worked first try once I knew the
field existed. The previous failure mode was a confusing "no Cargo.toml"
error from the workspace root, which a one-line note in the docs would
solve.