docs: add router callable API

[vibhor-aggr]

Summary

Closes #1151.

Adds API documentation for the callable router(req, res, [next]) middleware interface in both the Express 4.x and 5.x Router API pages.

The new section documents:

• the req, res, and optional next parameters
• that Express uses this callable form when routers are mounted with app.use() or router.use()
• how control and errors are passed through next() / next(err)

Verification

• npx prettier --check src/content/api/4x/api/router/index.mdx src/content/api/5x/api/router/index.mdx
• npx cspell --no-progress src/content/api/4x/api/router/index.mdx src/content/api/5x/api/router/index.mdx
• npm run lint -- src/content/api/4x/api/router/index.mdx src/content/api/5x/api/router/index.mdx
• git diff --check
• npm run build

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