Add input group example to the docs

[vanillajonathan]

Add an example to the documentation when using both .input-group-prepend and .input-group-append together to create an input group with "the extra" between two inputs.

Example 1

<div class="input-group">
  <input class="form-control" type="text" />
  <div class="input-group-prepend input-group-append">
    <div class="input-group-text">@</div>
  </div>
  <select class="form-control"></select>
</div>

Example 2

<div class="input-group">
  <input class="form-control" type="date" />
  <div class="input-group-prepend input-group-append">
    <div class="input-group-text">–</div>
  </div>
  <input class="form-control" type="date" />
</div>

[ysds]

Duplicate of #25120