Instructions for adding a new query

Summary of issue

We have a good start on documenting adding a new query: https://rustc-dev-guide.rust-lang.org/query.html#adding-a-new-kind-of-query. However, I think there's room for greater clarity.

☞ After reading this section, I know where to define a query. I want to know where and how to implement one.

Details

Currently, this section of the guide has two summaries of the process:

So suppose you want to add a new kind of query, how do you do so? Well, defining a query takes place in two steps:

1. first, you have to specify the query name and arguments; and then,
2. you have to supply query providers where needed.

and

So, to add a query:

1. Add an entry to rustc_queries! using the format above.
2. Link the provider by modifying the appropriate provide method; or add a new one if needed and ensure that rustc_driver is invoking it.

Where are these providers?
How do I find "the appropriate provide method" for a new query? Do I always need to create a new one for a new query? Or can I sometimes reuse ("modify") an existing one?

The section immediately before this one gives information on Providers, but does not say how to add a new one.

Start with the adding-a-new-kind-of-query and Providers sections of query.html, then trace the rustc_queries! entry, provider methods, and rustc_driver invocation mentioned there. Update the guide to identify provider locations and explain when to modify an existing provide method versus add one, including how to ensure it is invoked.