Fix Issue #295: Be more explicit about the need for UQTestFuns.

[damar-wicaksono]

• A statement of need for UQTestFuns is made more explicit in the landing page with a link to a more detailed discussion.
• Minor edit on the "About UQ Test Functions" edit.
• Fix Issue Issue on page /test-functions/speed-reducer-shaft.html #289: remove an unnecessary sentence.
• Fix Issue Issue on page /test-functions/bratley1992b.html #288: fix missing subscript in a notation.

This PR should resolve Issues #288, #289, and #295.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (7e4ac72) 98.85% compared to head (225ee53) 98.85%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #300   +/-   ##
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  Coverage   98.85%   98.85%           
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  Files          56       56           
  Lines        1916     1916           
  Branches      139      139           
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  Hits         1894     1894           
  Misses         11       11           
  Partials       11       11           

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