custom-theme/footer.html

<footer class="row">
    <div class="col-xs-12">
        {% if config.copyright %}
            <p class="copyright">{{ config.copyright }}</p>
        {% endif %}
    </div>
    <div class="col-xs-12">
        <div class="logos">
            <a href="https://www.apache.org/">
                <img src="/img/asf_logo_wide_small.png" alt="Apache" title="Apache">
            </a>
            <p>
                Copyright © 2015-2021 The Apache Software Foundation.<br>
                <small class="footnote">
                    Apache Mynewt, Mynewt, Apache, the Apache feather logo, and the Apache Mynewt
                    project logo are either registered trademarks or trademarks of the Apache
                    Software Foundation in the United States and other countries.
                </small>
            </p>
            <a href="{{ config.extra.slack_url }}">
                <img src="https://www.countit.com/images/add_to_slack.png" alt="Slack Icon" title="Join our Slack Community" />
            </a>
        </div>
    </div>
    <a href="https://www.apache.org/licenses/">
        <button class="button-footer-asf">
            License
        </button>
    </a>
    <a href="https://www.apache.org/foundation/sponsorship.html">
        <button class="button-footer-asf">
            Sponsorship
        </button>
    </a>
    <a href="https://www.apache.org/foundation/thanks.html">
        <button class="button-footer-asf">
            Thanks
        </button>
    </a>
    <a href="https://www.apache.org/security/">
        <button class="button-footer-asf">
            Security
        </button>
    </a>
    <a href="https://apache.org/events/current-event">
        <button class="button-footer-asf">
            ASF Events
        </button>
    </a>
</footer>