Add new placeholder component (#31859)

Co-authored-by: XhmikosR <xhmikosr@gmail.com>
Co-authored-by: Jaume Sala <jaumesala@gmail.com>
Co-authored-by: Mark Otto <markdotto@gmail.com>
Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>

site/data/sidebar.yml

@@ -75,6 +75,7 @@
     - title: Navbar
     - title: Offcanvas
     - title: Pagination
+    - title: Placeholders
     - title: Popovers
     - title: Progress
     - title: Scrollspy