Move mermaid integration code to assets

While they work where they are, anyone wanting to use this as a remote theme needs the files to be in [Jekyll’s assets directory](https://jekyllrb.com/docs/themes/#assets) so that they are available when the theme is fetched.

Author: mrmanc

_layouts/reveal.html

@@ -21,7 +21,7 @@
 	{% endfor %}
 {% endif %}
 {% if site.mermaid_diagrams %}
-		<link rel="stylesheet" href="{{ 'css/mermaid.css' | relative_url }}">
+		<link rel="stylesheet" href="{{ 'assets/css/mermaid.css' | relative_url }}">
 {% endif %}
 		<!-- For syntax highlighting -->
 		{% if site.highlight_style_sheet %}
@@ -123,7 +123,7 @@
 					{ src: '{{ site.reveal_path }}plugin/zoom-js/zoom.js', async: true, condition: function() { return !!document.body.classList; } },
 					{% if site.mermaid_diagrams %}
 					{ src: 'https://cdn.jsdelivr.net/npm/[email protected]/dist/mermaid.min.js'},
-					{ src: '{{ 'js/mermaid/reveal-js-mermaid-plugin.js' | relative_url }}'},
+					{ src: '{{ 'assets/js/mermaid/reveal-js-mermaid-plugin.js' | relative_url }}'},
 					{% endif %}
 					{% if site.reveal_notes_server %}
 					{ src: '{{ site.reveal_path }}socket.io/socket.io.js', async: true },