Add an arrow icon for Remix link, with proper attribution

docs/_static/img/share-solid.svg

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+<svg aria-hidden="true" focusable="false" data-prefix="fas" data-icon="share" class="svg-inline--fa fa-share fa-w-16" role="img" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path fill="currentColor" d="M503.691 189.836L327.687 37.851C312.281 24.546 288 35.347 288 56.015v80.053C127.371 137.907 0 170.1 0 322.326c0 61.441 39.581 122.309 83.333 154.132 13.653 9.931 33.111-2.533 28.077-18.631C66.066 312.814 132.917 274.316 288 272.085V360c0 20.7 24.3 31.453 39.687 18.164l176.004-152c11.071-9.562 11.086-26.753 0-36.328z"></path></svg>

docs/_templates/footer.html

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+{% extends "!footer.html" %}
+
+{% block extrafooter %}
+    <br>
+    <a href="{{ pathto('credits-and-attribution') }}">Credits and attribution</a>.
+{% endblock %}

docs/credits-and-attribution.rst

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+.. This page is meant to be linked to from the footer.
+
+:orphan:
+
+#######################
+Credits and Attribution
+#######################
+
+Website icons
+=============
+
+.. |icon-share-solid| image:: _static/img/share-solid.svg
+.. _share icon: https://fontawesome.com/v5.15/icons/share?style=solid
+.. _Font Awesome Free License: https://fontawesome.com/license/free
+
++-------------------------+-----------------------------------------------------------------------+
+| Icon                    | Attribution                                                           |
++=========================+=======================================================================+
+| |icon-share-solid|      | - Source: `share icon`_ from Font Awesome 5.15.0.                     |
+|                         | - License: `Font Awesome Free License`_ (CC BY 4.0).                  |
++-------------------------+-----------------------------------------------------------------------+