Accessibility: the agile collective logo alt text

<div id="branding">
  <a href="/">
    <img src="/_assets/images/ac-logo-white-red-box.svg" alt="Agile Collective logo" height="126" width="459" decoding="async" class="header_logo">
    <span class="header__title">Agile Collective Staff Handbook</span>
  </a>
</div>

The alt text is repetitious in this context. Let's mark it decorative by making the alt text empty

<img src="/_assets/images/ac-logo-white-red-box.svg" alt="" height="126" width="459" decoding="async" class="header_logo">