docs: progress bar content enhancements

[laurenmrice]

Content enhancements

• The current progress bar component usage and style guidance has been reviewed from accessibility with requested updates.

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• Suggestions for content enhancements are provided in pdfs below:
  • Usage tab guidance: https://ibm.ent.box.com/file/953648079387?s=jvu3n241qwsynpuoit2cb6hq2byx3p7g
  • Style tab guidance: https://ibm.box.com/s/xjt4n00xi71tijvaj0fsdbiw0gtau942
  • Images with the success state need to be updated to used the $support-success color token.

[thyhmdo]

Changes for Usage tab

• Updated Code and Accessibility tabs
• Overview: changed loading spinners to loading
• Variants : Updated the content
• Determined progress bar: Updated the content
• Anatomy: Updated Anatomy image and content
• Alignment: Alignment to Text alignment; updated the content
• Content: Updated labels content
• Universal behaviors: Add image states; Updated the content and image for Error state
• Accessibility: Updated the content

Changes for Style tab

• Omit theme images