… in CI

The execution_graph README Quick Start dropped `?` on `add_node` and `set_input_value` (both return `Result`), so the `fn main()` example failed to compile with two E0308s. A reader's first copy-paste from the docs would not build, while the lib.rs doctest stayed correct because only it was exercised by CI.

To stop this drift recurring, include each crate's README.md as a `#[cfg(doctest)]` item so the existing `cargo test --doc --workspace` step compiles the Quick Start. The item exists only under `cfg(doctest)`, so it never appears in rendered docs and is invisible to clippy and normal builds. execution_tape's README already compiled; its guard is purely protective.