docs: update extending theme

[lucksp]

Motivation

While working though how to get a proper custom & extended theme #3473 I thought the documents could use a small massage.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Jest Unit Test
• Checked with example app

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation using yarn docs-build-api
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Additional context

[codecov]

Codecov Report

Merging #3482 (4595c47) into next (d1ac3fc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             next    #3482   +/-   ##
=======================================
  Coverage   78.52%   78.52%           
=======================================
  Files          87       87           
  Lines        1774     1774           
  Branches      790      790           
=======================================
  Hits         1393     1393           
  Misses        376      376           
  Partials        5        5           

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[arpitBhalla]

Thank you for the contribution 🎉