website: Fix docs for implied provider FS mirrors
website: Fix docs for implied provider FS mirrors

In addition to the directories previously listed, Terraform looks in the
CLI config directory ($HOME/.terraform.d/plugins on macOS/Linux/UNIX,
and %APPDATA%/terraform.d/plugins on Windows). List this in the
documentation for clarity.

We also add a note about the working directory relative "vendor"
location, ./terraform.d/plugins.

pdecat/terraform,tommynsong/terraform,tommynsong/terraform,ewbankkit/terraform,hashicorp/terraform,hashicorp/terraform,ewbankkit/terraform,apparentlymart/terraform,rnaveiras/terraform,pdecat/terraform,ewbankkit/terraform,apparentlymart/terraform,rnaveiras/terraform,tommynsong/terraform,pdecat/terraform,apparentlymart/terraform,ewbankkit/terraform,hashicorp/terraform,apparentlymart/terraform,pdecat/terraform,rnaveiras/terraform,apparentlymart/terraform