doc(loading): Updated the Usage & Style tab (carbon-design-system#4554)

* Loading doc - Usage & Style

* Fixed a typo

* Lauren's feedback

* Fixed few typos

* Fixed a small typo

* Fixed the GIF border

* Feedback

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Co-authored-by: Lauren Rice <43969356+laurenmrice@users.noreply.github.com>
Co-authored-by: kennylam <909118+kennylam@users.noreply.github.com>

Author: 3 people