website: Reconcile docs about plugin discovery and downloading
I'm attempting to keep things simple for normal users while making sure we've
got the full behavior written down somewhere for plugin developers.
This commit doesn't stand alone; it's paired with a commit in the
terraform-website repo, to deal with some related content in the "extend"
section.
This mechanism for configuring plugins is now deprecated, since it's not
capable of declaring plugin versions. Instead, we recommend just placing
plugins into a particular directory, which is now documented on the
main providers documentation page and linked from the more detailed docs
on plugins in general.
* update plugin/provider to make clear this section isn't needed for regular use
* add some links and notes about getting started
* remove the mention of binaries... I 'm not sure it's needed yet
* 'Installing Terraform Providers' section
* sometimes I can't words good
* move the 'installing providers' block
* cleanup of terms
* copy that update to plugins/provider too