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 #289: remove an unnecessary sentence.
• Fix Issue #288: fix missing subscript in a notation.

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

[codecov[bot]]

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

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

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