accordion(1つ目のcardクラス)

accordion.html

<div class="card">
      <div class="card-header" id="heading1">
          <h2 class="mb-0">
              <button class="btn btn-link" type="button" data-toggle="collapse" data-target="#collapse1" aria-expanded="true" aria-controls="collapse1">アコーディオン表示その1</button>
          </h2>
      </div>

      <div id="collapse1" class="collapse show" aria-labelledby="heading1" data-parent="#accordionExample">
          <div class="card-body">
              カードの中身1
          </div>
      </div>
</div>