Add missing chevron on previous-next buttons (Qiskit#237)

This PR adds missing chevron svg file used in footer Previous/Next
buttons (original image accidentally removed in Qiskit#138 ).
It also updates references from `chevron-right-orange` to
`chevron-right-purple`, matching the fill colour used.

Updated footer rendering:

![image](https://user-images.githubusercontent.com/25207344/228622405-0e8a0ac2-f1b2-44af-be2c-85ee4ac48d52.png)

Fixes Qiskit#189

qiskit_sphinx_theme/footer.html

@@ -16,10 +16,10 @@
   <!-- NEXT/PREVIOUS BUTTONS -->
   <div class="rst-footer-buttons" role="navigation" aria-label="footer navigation">
     {% if next %}
-      <a href="{{ next.link|e }}" class="btn btn-neutral float-right" title="{{ next.title|striptags|e }}" accesskey="n" rel="next">{{ _('Next') }} <img src="{{ pathto('_static/images/chevron-right-orange.svg', 1) }}" class="next-page"></a>
+      <a href="{{ next.link|e }}" class="btn btn-neutral float-right" title="{{ next.title|striptags|e }}" accesskey="n" rel="next">{{ _('Next') }} <img src="{{ pathto('_static/images/chevron-right-purple.svg', 1) }}" class="next-page"></a>
     {% endif %}
     {% if prev %}
-      <a href="{{ prev.link|e }}" class="btn btn-neutral" title="{{ prev.title|striptags|e }}" accesskey="p" rel="prev"><img src="{{ pathto('_static/images/chevron-right-orange.svg', 1) }}" class="previous-page"> {{ _('Previous') }}</a>
+      <a href="{{ prev.link|e }}" class="btn btn-neutral" title="{{ prev.title|striptags|e }}" accesskey="p" rel="prev"><img src="{{ pathto('_static/images/chevron-right-purple.svg', 1) }}" class="previous-page"> {{ _('Previous') }}</a>
     {% endif %}
   </div>