docs: Force sections to be full-width

Otherwise there's this weird *gap*, where the sections are narrow even
tho there's plenty of space?

So you have this screen layout:

```table
| sidebar | text        |
| sidebar | narr        |
| sidebar | ower        |
| sidebar | than        |
| sidebar | need        |
| sidebar | ed          |
```

For some gosh-forsaken reason.

doc_src/python_docs_theme/static/pydoctheme.css

@@ -35,6 +35,10 @@ div.section {
        but I have no idea how to make the tables themselves scrollable
      */
     overflow-y: auto;
+    /* This avoids leaving the actual text narrower than necessary
+       in certain widths.
+     */
+    width: 100%;
 }
 
 div.related a:hover {