docs: fix axum examples and doctests

[kieled]

Summary

• Update the doctest examples in aide::axum, aide::redoc, aide::swagger, and aide::scalar to avoid calling axum::serve, since it is feature-gated in axum.
• Use plain IntoResponse for the non-documented /api.json handler in the aide::axum docs example.

Test Plan

• cargo test -p aide --doc --all-features

(Originally part of #285; split out per review feedback.)

[jplatte]

Why not add a dev-dependency on axum with the tokio feature? Then axum::serve can be used in doctests.

[kieled]

axum::serve isn't enabled by tokio alone. It also needs http1 or http2 / default features. Most importantly, I avoided adding extra axum features only for doctests because aide depends on axum with default-features = false, and making doctests pass via a beefier dev-dep can mask “docs don’t compile” cases for users running minimal axum features.

[jplatte]

beefier dev-dep can mask “docs don’t compile” cases for users running minimal axum features

I don't think this should be a serious concern nowadays. rustc has supported "this item exists, but is under a disabled cfg" hints for a couple versions now.

[kieled]

Fair point — updated the PR to add axum as a dev-dependency with tokio and http1 features instead.