Add option html_sourcelink_txt

doc/config.rst

@@ -870,6 +870,13 @@ that use Sphinx's HTMLWriter class.
 
    .. versionadded:: 0.6
 
+.. confval:: html_sourcelink_txt
+
+   If true, ``.txt`` is appended to source links (see
+   :confval:`html_show_sourcelink`).  Default is ``True``.
+
+   .. versionadded:: 1.5
+
 .. confval:: html_use_opensearch
 
    If nonempty, an `OpenSearch <http://www.opensearch.org/Home>`_ description file will be