Docs: Add hidden user agent extension env var info to terraform website

[modular-magician]

Add hidden user agent extension env var information to terraform website document.

See undocumented environment variable check for GOOGLE_TERRAFORM_USER_AGENT_EXTENSION in the providerConfigure function.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

See corresponding issue on the terraform-provider-google repository.

Derived from GoogleCloudPlatform/magic-modules#7470