diff() is a manipulation function, not a display function

**Describe the bug**
In the docs, documentation for `diff()` is found under the _difference_ topic, filed under the top-level heading, _display_. However, diffing is not concerned with the display of dates but of manipulating them, ergo it is misclassified.

**Expected behavior**
_Difference_ topic should be filed under _manipulation_ heading.

**Information**
n/a

Start in the Day.js documentation where the diff() documentation is listed under the difference topic and display heading. Move the difference topic under the manipulation heading, then verify that the documentation navigation and topic placement reflect the issue's expected behavior.

Written by the indexing model from the issue text.