Media storage provider functionality is not documented

"Media storage providers" are external modules that can be used to store and fetch media from sources other than the local filesystem. One popular example is https://github.com/matrix-org/synapse-s3-storage-provider/, which allows you to store media on Amazon S3, and fetch it when requested.

The [Synapse documentation](https://github.com/element-hq/synapse/tree/develop/docs) doesn't mention this functionality at all currently, which makes it unclear to Synapse developers that it exists, and unknown to third-party developers that they can develop these things.

A media storage provider module must implement the [`StorageProvider` class](https://github.com/element-hq/synapse/blob/bd8d8865fba3d6c68c9a4d698f42549820fdf12c/synapse/media/storage_provider.py#L42) (`store_file` and `fetch_media`). Media storage providers installed into the Python environment [can be configured](https://element-hq.github.io/synapse/v1.106/usage/configuration/config_documentation.html#media_storage_providers) through Synapse's config file.

Start in the Synapse docs directory and read synapse/media/storage_provider.py, especially the StorageProvider class and its store_file and fetch_media methods. Check the existing media_storage_providers configuration documentation for placement and terminology. Done means developers can discover the provider functionality, understand how to implement a provider, and configure an installed module.

Written by the indexing model from the issue text.