Update sphinx theme

Use the default readthedocs theme instead of our custom theme. This
ensure we use the same theme for both 5.x and 6.x documentation. We
also benefit from niceties included in the latest readthedocs theme
that our custom theme doesn't provide (e.g better mobile layout).

Author: dannycolin

docs/en/images/bugzilla-logo-white.svg

@@ -1,21 +1,4 @@
-@import 'default.css';
-
-dt { font-weight: bold; }
-
-/* Custom roles */
-.param    { font-weight: bold; }
-.paramval { font-family: monospace; }
-.group    { font-family: monospace; }
-.field    { font-weight: bold; }
-.command  { font-family: monospace; font-size: 130% }
-
-.admonition-todo {
-  background-color: lightpink;
-  border: 2px darkred solid;
-}
-
-/* Make Buggie's antenna not take up so much space */
-.logo {
-  display: block;
-  margin-top: -20px;
-}
+.wy-side-nav-search .wy-dropdown > a img.logo,
+.wy-side-nav-search > a img.logo {
+  min-width: 150px;
+  width: 350px;

docs/en/rst/_static/bugzilla.css

@@ -128,12 +128,31 @@
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'default'
+html_theme = 'sphinx_rtd_theme'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
 # documentation.
-#html_theme_options = {}
+html_theme_options = {
+    #'analytics_id': 'G-XXXXXXXXXX',  #  Provided by Google in your dashboard
+    #'analytics_anonymize_ip': False,
+    'logo_only': True,
+    'display_version': True,
+    'prev_next_buttons_location': 'bottom',
+    'style_external_links': False,
+    'vcs_pageview_mode': '',
+    #'style_nav_header_background': 'white',
+    # Toc options
+    'collapse_navigation': True,
+    'sticky_navigation': True,
+    'navigation_depth': 4,
+    'includehidden': True,
+    'titles_only': False
+}
+
+html_css_files = [
+    "bugzilla.css"
+]
 
 # Add any paths that contain custom themes here, relative to this directory.
 #html_theme_path = []
@@ -144,12 +163,11 @@
 
 # A shorter title for the navigation bar.  Default is the same as html_title.
 #html_short_title = None
-
-html_style = "bugzilla.css"
+#html_style = "bugzilla.css"
 
 # The name of an image file (relative to this directory) to place at the top
 # of the sidebar.
-html_logo = "../images/bugzlla.png"
+html_logo = "../images/bugzlla-logo-white.svg"
 
 # The name of an image file (within the static path) to use as favicon of the
 # docs.  This file should be a Windows icon file (.ico) being 16x16 or 32x32