docs: add a runnable config inspection example

[tisonkun]

Summary

• add a root-level, non-published serde-shape-examples workspace crate with an inspect_config example that prints an environment-variable reference for a fictional application configuration;
• link the runnable example from the README with a relative source link;
• remove the old configenv integration fixture because its Serde behaviors already have focused derive coverage.

Design notes

The example has its own local crate because showcase dependencies and targets should not affect the published serde-shape manifest. As a workspace member, it is still compiled and linted by the existing all-target workflow.

Environment naming and shape support remain example policy rather than public library API. The walker handles the small set used by the fictional model, detects recursive definitions, and rejects representations it cannot map faithfully.

The example contains no names, paths, or configuration details from private downstream repositories.

Validation

• cargo run --package serde-shape-examples --example inspect_config
• cargo x build
• cargo x test
• cargo x lint
• cargo x package