[chore][fileprovider] Add README file (#12506)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

Use mdatagen for the file provider. This switches the package test to be
generated and adds an autogenerated section to the readme.

I've also populated the readme with basic usage information.

confmap/provider/fileprovider/README.md

@@ -0,0 +1,27 @@
+# File Provider
+
+<!-- status autogenerated section -->
+| Status        |           |
+| ------------- |-----------|
+| Stability     | [stable]  |
+| Distributions | [core], [contrib], [k8s], [otlp] |
+| Issues        | [![Open issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector?query=is%3Aissue%20is%3Aopen%20label%3Aprovider%2Ffileprovider%20&label=open&color=orange&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector/issues?q=is%3Aopen+is%3Aissue+label%3Aprovider%2Ffileprovider) [![Closed issues](https://img.shields.io/github/issues-search/open-telemetry/opentelemetry-collector?query=is%3Aissue%20is%3Aclosed%20label%3Aprovider%2Ffileprovider%20&label=closed&color=blue&logo=opentelemetry)](https://github.com/open-telemetry/opentelemetry-collector/issues?q=is%3Aclosed+is%3Aissue+label%3Aprovider%2Ffileprovider) |
+
+[stable]: https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/component-stability.md#stable
+[core]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol
+[contrib]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
+[k8s]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-k8s
+[otlp]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-otlp
+<!-- end autogenerated section -->
+
+## Overview
+
+The File Provider takes paths to files and reads their contents as YAML to provide configuration to the Collector.
+
+## Usage
+
+The scheme for this provider is `file`. Usage looks like the following:
+
+```text
+file:/path/to/file.yaml
+```

confmap/provider/fileprovider/generated_package_test.go

@@ -1,5 +1,4 @@
-// Copyright The OpenTelemetry Authors
-// SPDX-License-Identifier: Apache-2.0
+// Code generated by mdatagen. DO NOT EDIT.
 
 package fileprovider
 

confmap/provider/fileprovider/metadata.yaml

@@ -0,0 +1,8 @@
+type: file
+github_project: open-telemetry/opentelemetry-collector
+
+status:
+  class: provider
+  stability:
+    stable: [provider]
+  distributions: [core, contrib, k8s, otlp]

confmap/provider/fileprovider/provider.go

@@ -1,6 +1,8 @@
 // Copyright The OpenTelemetry Authors
 // SPDX-License-Identifier: Apache-2.0
 
+//go:generate mdatagen metadata.yaml
+
 package fileprovider // import "go.opentelemetry.io/collector/confmap/provider/fileprovider"
 
 import (