Migrate Provider Metadata Documentation Under Plugin Documentation

## Description

The Terraform CLI internals documentation has a page describing [Provider Metadata](https://www.terraform.io/internals/provider-meta), a lesser-known feature where provider developers who also happen to be module developers can support module-level "provider" configuration.

Since this functionality was introduced, Terraform CLI has it included with 1.0 compatibility promises, so it is not intended to go anywhere for awhile. It is no longer considered an "experimental" feature. Both current major protocol versions support the data and its usable with both terraform-plugin-sdk/v2 and terraform-plugin-framework.

Given that this functionality requires provider implementation and that additional providers may benefit from it, this documentation should likely be promoted to the plugin/ section of the website where each development framework can document the implementation specifics.

Start with the existing Provider Metadata page at terraform.io/internals/provider-meta and inspect the plugin/ documentation structure in this repository. Move or reorganize the page under plugin documentation, preserving the compatibility context and adding implementation-specific guidance for terraform-plugin-sdk/v2 and terraform-plugin-framework. Done means provider developers can find the feature in the plugin documentation.

Written by the indexing model from the issue text.