feat: added accentB background for page banner

[mehaknasir]

Description

Feature Ticket: https://2u-internal.atlassian.net/browse/INF-599
Figma designs supporting required change: https://www.figma.com/file/LVFnO9oRHYiamoicZuXDAC/Discussions?node-id=6884%3A261359

Page banners do not accept background color classes hence we need to add a new variant that can help achieve a design requirement in discussion MFE.

• accentB color background is added with variant name accentB.
• documentation is updated to state available variants
• test cases added to ensure the required color class is added in the background.

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[codecov]

Codecov Report

Base: 90.36% // Head: 90.36% // No change to project coverage 👍

Coverage data is based on head (e7662fc) compared to base (3dad628).
Patch has no changes to coverable lines.

❗ Current head e7662fc differs from pull request most recent head 33a6525. Consider uploading reports for the commit 33a6525 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1688   +/-   ##
=======================================
  Coverage   90.36%   90.36%           
=======================================
  Files         208      208           
  Lines        3580     3580           
  Branches      838      838           
=======================================
  Hits         3235     3235           
  Misses        330      330           
  Partials       15       15           

Impacted Files	Coverage Δ
src/PageBanner/index.jsx	91.66% <ø> (ø)

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[adamstankiewicz]

Thanks for the updates! LGTM.

[edx-semantic-release]

🎉 This PR is included in version 20.15.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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