router: hide low-level route helpers from rustdoc

[Yong-yuan-X]

Summary

• hide all ten stringly-typed Router::route_* helpers from rustdoc while keeping them public for generated code
• direct users toward the generated <Service>Ext::register API
• replace manual routing examples and references in public documentation
• clarify Router::with_spec behavior without hiding the typed API

This focuses on the small scope described in #91: hiding the low-level stringly-typed route helpers from rustdoc and updating the docs to point users toward generated service registration.

Runtime behavior and API compatibility are unchanged.

Fixes #91

Testing

• cargo fmt -p connectrpc -- --check
• cargo clippy -p connectrpc --all-features --lib -- -D warnings
• cargo test -p connectrpc --lib
• cargo doc -p connectrpc --all-features --no-deps
• cargo check -p multiservice-example --all-features
• cargo check -p eliza-example --all-features

[github-actions]

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