Spinix: use html_css_files html_js_files

We are using an incorrect way to add additional CSS files.

This is causing a number of problems.

 * Unable to update sphinx-rtd-theme, due to no CSS styling
 * Issue#146 Copy button for code (button not displayed as expected)
 * Extensions that contribute CSS not working

The documentation for Spinix confirming this commit uses the correct method:

https://docs.readthedocs.io/en/stable/guides/adding-custom-css.html#how-to-add-custom-css-or-javascript-to-sphinx-documentation

source/_templates/layout.html

@@ -15,10 +15,6 @@
     {% endif %}
     {% endfor %}
 {% endblock %}
-{% block scripts %}
-    {{ super() }}
-    <script src="{{ pathto('_static/dialog.js', 1) }}"></script>
-{% endblock %}
 {% block extrabody %}
     {{ super() }}
     {% if hasdoc(pagename) %}

source/conf.py

@@ -201,12 +201,17 @@
     'titles_only': False
 }
 
+html_css_files = [
+    'theme_overrides.css',  # override wide tables in RTD theme
+    'gh-fork-ribbon.css',
+    'spinaldoc.css'
+]
+
+html_js_files = [
+    'dialog.js'
+]
+
 html_context = {
-    'css_files': [
-        '_static/theme_overrides.css',  # override wide tables in RTD theme
-        '_static/gh-fork-ribbon.css',
-        '_static/spinaldoc.css'
-    ],
     'head_meta': {
         'robots': os.getenv('sphinx_html_context_head_meta_robots', None)
     },