Remove data_files from declarative config example

According to the following references:

- https://setuptools.pypa.io/en/latest/references/keywords.html
- https://packaging.python.org/guides/distributing-packages-using-setuptools/#package-data

the usage of ``data_files`` is deprecated.

Therefore we should avoid having it in the main configuration example
for declarative config.

changelog.d/2932.doc.1.rst

@@ -0,0 +1,2 @@
+Removed the deprecated ``data_files`` option from the example in the
+declarative configuration docs -- by :user:`abravalheri`

changelog.d/2932.doc.2.rst

@@ -0,0 +1,3 @@
+Change type of ``data_files`` option from ``dict`` to ``section`` in
+declarative configuration docs (to match previous example) -- by
+:user:`abravalheri`

docs/userguide/declarative_config.rst

@@ -63,13 +63,6 @@ boilerplate code in some cases.
         src.subpackage1
         src.subpackage2
 
-    [options.data_files]
-    /etc/my_package =
-        site.d/00_default.conf
-        host.d/00_default.conf
-    data = data/img/logo.png, data/svg/icon.svg
-    fonts = data/fonts/*.ttf, data/fonts/*.otf
-
 Metadata and options are set in the config sections of the same name.
 
 * Keys are the same as the keyword arguments one provides to the ``setup()``
@@ -222,7 +215,7 @@ package_data             section                                              [#
 exclude_package_data     section
 namespace_packages       list-comma
 py_modules               list-comma                            34.4.0
-data_files               dict                                 40.6.0
+data_files               section                              40.6.0          [#opt-4]_
 =======================  ===================================  =============== =========
 
 **Notes**:
@@ -250,6 +243,9 @@ data_files               dict                                 40.6.0
 
    The ``find_namespace:`` directive is supported since Python >=3.3.
 
+.. [#opt-4] ``data_files`` is deprecated and should be avoided.
+   Please check :doc:`/userguide/datafiles` for more information.
+
 
 Compatibility with other tools
 ==============================