Docs: Add Azure, Filesystem, Imgur and S3

Signed-off-by: Philip Molares <philip.molares@udo.edu>

docs/content/media/azure.md

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+# Azure Blob Storage
+
+You can use [Microsoft Azure Blob Storage](https://azure.microsoft.com/services/storage/blobs/) to handle your image uploads in HedgeDoc.
+
+All you need to do is to get the [connection string](https://docs.microsoft.com/azure/storage/common/storage-account-keys-manage) for your storage account and create a storage container with public access set to 'blob'.
+
+It's possible to create the container with the [Azure CLI](https://docs.microsoft.com/en-us/cli/azure/install-azure-cli), using your connection string, with the following command:
+
+```
+az storage container create --name <NAME> --public-access blob --connection-string "<CONNECTION_STRING>"
+```
+
+You can of course also create the container in the Azure portal if you prefer.
+
+Then you just add the following lines to your configuration:  
+<small>(with the appropriate substitution for `<CONNECTION_STRING>` and `<NAME>` of course)</small>
+```
+HD_MEDIA_BACKEND="azure"
+HD_MEDIA_BACKEND_AZURE_CONNECTION_STRING="<CONNECTION_STRING>"
+HD_MEDIA_BACKEND_AZURE_CONTAINER="<NAME>"
+```

docs/content/media/filesystem.md

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+# Filesystem
+
+You can use your local filesystem to handle your image uploads in HedgeDoc.
+
+You just add the following lines to your configuration:  
+<small>(with the appropriate substitution for `<DIRECTORY>` of course)</small>
+```
+HD_MEDIA_BACKEND="filesystem"
+HD_MEDIA_BACKEND_FILESYSTEM_UPLOAD_PATH="<DIRECTORY>"
+```
+
+Please make sure that the user that runs HedgeDoc is able to write to the uploads directory. 

docs/content/media/imgur.md

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+# Imgur
+
+You can use [Imgur](https://imgur.com) to handle your image uploads in HedgeDoc.
+
+All you need for that is to register an application with Imgur and get a client id:
+
+1. Create an account on imgur.com and log in.
+2. Go to <https://api.imgur.com/oauth2/addclient>
+3. Fill out the form and choose "Anonymous usage without user authorization" as the authorization type.
+4. Imgur will then show you your client id.
+
+Then you just add the following lines to your configuration:  
+<small>(with the appropriate substitution for `<IMGUR_CLIENT_ID>` of course)</small>
+```
+HD_MEDIA_BACKEND="imgur"
+HD_MEDIA_BACKEND_IMGUR_CLIENT_ID="<IMGUR_CLIENT_ID>"
+```
+
+All uploads are saved in the `media_uploads` database table and contain the deletion token ([see here](https://apidocs.imgur.com/#949d6cb0-5e55-45f7-8853-8c44a108399c)) in the column `backendData`.

docs/content/media/s3.md

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+# S3-compatible
+
+You can use [Amazon S3](https://aws.amazon.com/s3/) or any other S3-compatible storage (like [MinIO](https://min.io) or [Ceph Object Gateway](https://docs.ceph.com/en/latest/radosgw/)) to handle your image uploads in HedgeDoc.
+
+Your S3 bucket must be configured to be writeable.
+
+You just add the following lines to your configuration:  
+<small>(with the appropriate substitution for `<ACCESS_KEY>`, `<SECRET_KEY>`, `<BUCKET>`, and `<ENDPOINT>` of course)</small>
+```
+HD_MEDIA_BACKEND="s3"
+HD_MEDIA_BACKEND_S3_ACCESS_KEY="<ACCESS_KEY>"
+HD_MEDIA_BACKEND_S3_SECRET_KEY="<SECRET_KEY>"
+HD_MEDIA_BACKEND_S3_BUCKET="<BUCKET>"
+HD_MEDIA_BACKEND_S3_ENDPOINT="<ENDPOINT>"
+```
+
+If you use Amazon S3, `<ENDPOINT>` should contain your [Amazon Region](https://docs.aws.amazon.com/general/latest/gr/s3.html).
+
+For example: If your Amazon Region is `us-east-2`, your endpoint `<ENDPOINT>` should be `s3.us-east-2.amazonaws.com`.