v4: Social meta tags (#20825)

* descriptions for getting started pages

* descriptions for layout

* add content page descriptions

* more descriptions, updates to some existing ones

* correct site url

* add social stuff to config for twitter cards

* add twitter meta tags; use large image for homepage and regular card for all others

* add the assets

* more site config

* more social shiz to partial, remove existing meta for the partial, remove page title from homepage for simpler if statements

docs/layout/flexbox-grid.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Flexbox grid system
+description: Documentation and examples for using Bootstrap's optional flexbox grid system.
 group: layout
 ---
 

docs/layout/grid.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Grid system
+description: Documentation and examples for using Bootstrap's powerful, responsive, and mobile-first grid system.
 group: layout
 ---
 

docs/layout/media-object.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Media object
+description: Documentation and examples for Bootstrap's media object to construct highly repetitive components like blog comments, tweets, and the like.
 group: layout
 ---
 

docs/layout/overview.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Overview
+description: Components and options for laying out your Bootstrap project, including wrapping containers, a powerful grid system, a flexible media object, and responsive utility classes.
 group: layout
 redirect_from: "/layout/"
 ---

docs/layout/responsive-utilities.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Responsive utilities
+description: Use responsive display utility classes for showing and hiding content by device, via media query.
 group: layout
 ---