Handbook: Always show heading link indicator

Only showing it on hover can results in folks not knowing it's there. Hovering isn't a UI concept on mobile devices either, which makes the problem worse. Hovering is best used for design enhancements rather than functionality.

See https://ux.stackexchange.com/questions/20866/ui-hover-focus-to-reveal-controls
See https://ux.stackexchange.com/questions/82015/how-do-users-know-to-hover-over-elements

source/wp-content/plugins/handbook/inc/table-of-contents.php

@@ -10,9 +10,8 @@ class WPorg_Handbook_TOC {
 	protected $styles = '<style>
 		.toc-jump { text-align: right; font-size: 12px; }
 		.toc-heading a { color: inherit; font-weight: inherit; margin-left: -25px; text-decoration: none !important; }
-		.toc-heading a:before { visibility: hidden; vertical-align: middle; margin-top: -5px; margin-right: 5px; }
+		.toc-heading a:before { vertical-align: middle; margin-top: -5px; margin-right: 5px; }
 		.toc-heading:target a:before { margin-left: -8px; margin-right: 13px; }
-		.toc-heading a:hover:before, .toc-heading a:focus:before { visibility: visible; }
 	</style>';
 
 	/**