PR #34934 switched the `getting-started` docs example from using the
index of a product in the `products` array to using the product's ID
property for indentifiying each product in the `ProductDetailsComponent`
component. However, some necessary changes in the example code and the
`start-routing.md` guide were missed in #34934, resulting in broken
instructions for the readers (see #40189).
This commit is essentially a follow-up to #34934, making the remaining
changes in the example code and the guide instructions.
Fixes#40189
PR Close#40197