Prepend anchor IDs with a string to prevent them from starting with a…

… number

src/class-wpdtrt-anchorlinks-plugin.php

@@ -157,7 +157,7 @@ public function render_headings_as_anchors( string $content ) : string {
 
 			// class is also used by $this->render_headings_in_sections().
 			$heading->setAttribute( 'class', 'wpdtrt-anchorlinks__anchor' );
-			$heading_id = sanitize_title( $heading->nodeValue ); // phpcs:ignore
+			$heading_id = 'section-' . sanitize_title( $heading->nodeValue ); // phpcs:ignore
 			$heading->setAttribute( 'id', $heading_id );
 			$heading->setAttribute( 'tabindex', '-1' );