Document the new NewLoggingHTTPTransport and NewSubsystemLoggingHTTPTransport

[detro]

SDK version

2.19.0

Use-cases

The work in #1006 involves creating a feature that replaces a previously undocumented one: the NewTransport implementation of an http.RoundTripper, hosted in the helper/logging package.

The new transport is built around tflog logging library (https://github.com/hashicorp/terraform-plugin-log), and it requires website documentation to introduce it, explain why we deprecate the old one, and how to use it in tandem with tflog.

Proposal

Write a guide documentation to explain:

1. why using the legacy NewTransport is bad idea
2. what the new Transport(s) offer
3. how to correctly configure and pass context.Context to the transport via requests, to control logging
4. how to use NewSubsystemLoggingHTTPTransport to log via a subsystem logger, instead of the provider root logger used by NewLoggingHTTPTransport

The documentation should probably sit under the ? section of website/docs/plugin/sdkv2.

The documentation should definitely not try to cover the same content of https://www.terraform.io/plugin/log, and instead focus on the little amount of change necessary to use the new NewLoggingHTTPTransport, and then send to the Logging documentation for details.

References

Depends on: #1006

[github-actions]

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