Rewrite `usage/configuration.rst` (#12537)

- Remove now-removed configuration values
- Use obvious 'unset' defaults for ``project`` and ``author``
- Prefer 'e.g.' to 'ex.'
- Allow non-list sequence types in various configuration values
- Add types and defaults to every confval directive

tests/roots/test-root/conf.py

@@ -17,7 +17,10 @@ jsmath_path = 'dummy.js'
 
 templates_path = ['_templates']
 
-source_suffix = ['.txt', '.add', '.foo']
+source_suffix = {
+    '.txt': 'restructuredtext',
+    '.foo': 'foo',
+}
 
 project = 'Sphinx <Tests>'
 copyright = '1234-6789, copyright text credits'
@@ -68,7 +71,7 @@ latex_elements = {
   shadowrule=1pt,
   shadowsep=10pt,
   shadowsize=10pt,
-  div.topic_border-width=2pt,% alias to shadowrule 
+  div.topic_border-width=2pt,% alias to shadowrule
   div.topic_padding=6pt,% alias to shadowsep
   div.topic_box-shadow=5pt,% overrides/alias shadowsize
 %