doc: Add DatasourceScaleway documentation (canonical#4773)

Add the missing Scaleway datasource documentation to the list of
datasources of the reference section.

doc/rtd/reference/datasources.rst

@@ -60,6 +60,7 @@ The following is a list of documentation for each supported datasource:
    datasources/oracle.rst
    datasources/ovf.rst
    datasources/rbxcloud.rst
+   datasources/scaleway.rst
    datasources/smartos.rst
    datasources/upcloud.rst
    datasources/vmware.rst

doc/rtd/reference/datasources/scaleway.rst

@@ -0,0 +1,56 @@
+.. _datasource_scaleway:
+
+Scaleway
+********
+`Scaleway`_ datasource uses data provided by the Scaleway metadata service
+to do initial configuration of the network services.
+
+The metadata service is reachable at the following addresses :
+
+* IPv4: ``169.254.42.42``
+* IPv6: ``fd00:42::42``
+
+Configuration
+=============
+Scaleway datasource may be configured in system configuration
+(in `/etc/cloud cloud.cfg`) or by adding a file with the .cfg suffix containing
+the following information in the `/etc/cloud.cfg.d` directory::
+
+ datasource:
+   Scaleway:
+     retries: 3
+     timeout: 10
+     max_wait: 2
+     metadata_urls:
+       - alternate_url
+
+* ``retries``
+
+  Controls the maximum number of attempts to reach the metadata service.
+
+* ``timeout``
+
+  Controls the number of seconds to wait for a response from the metadata
+  service for one protocol.
+
+* ``max_wait``
+
+  Controls the number of seconds to wait for a response from the metadata
+  service for all protocols.
+
+* ``metadata_urls``
+
+  List of additional URLs to be used in an attempt to reach the metadata
+  service in addition to the existing ones.
+
+User Data
+=========
+
+cloud-init fetches user data using the metadata service using the `/user_data`
+endpoint. Scaleway's documentation provides a detailed description on how to
+use  `userdata`_. One can also interact with it using the `userdata api`_.
+
+
+.. _Scaleway: https://www.scaleway.com
+.. _userdata: https://www.scaleway.com/en/docs/compute/instances/api-cli/using-cloud-init/
+.. _userdata api: https://www.scaleway.com/en/developers/api/instance/#path-user-data-list-user-data