docs: Add Go Code Review Comments and allow the use of testify (#6984)

Inspired by
https://github.com/open-telemetry/opentelemetry-go-contrib/pull/7522#discussion_r2190580677

I hope it will help new contributors. Also I want to codify that we are
OK when one uses `testify`, but I do not also want to force it (e.g.
https://github.com/open-telemetry/opentelemetry-go-contrib/pull/7522 was
not using it).

I think that as long as we use the standard library, `testify`, `go-cmp`
we are fine (and should not accept any other libraries than those).

CONTRIBUTING.md

@@ -233,6 +233,10 @@ For a non-comprehensive but foundational overview of these best practices
 the [Effective Go](https://golang.org/doc/effective_go.html) documentation
 is an excellent starting place.
 
+We also recommend following the
+[Go Code Review Comments](https://go.dev/wiki/CodeReviewComments)
+that collects common comments made during reviews of Go code.
+
 As a convenience for developers building this project the `make precommit`
 will format, lint, validate, and in some cases fix the changes you plan to
 submit. This check will need to pass for your changes to be able to be
@@ -586,6 +590,10 @@ See also:
 
 ### Testing
 
+We allow using [`testify`](https://github.com/stretchr/testify) even though
+it is seen as non-idiomatic according to
+the [Go Test Comments](https://go.dev/wiki/TestComments#assert-libraries) page.
+
 The tests should never leak goroutines.
 
 Use the term `ConcurrentSafe` in the test name when it aims to verify the