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/getting-started/accessibility.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Accessibility
+description: Learn how Bootstrap supports common web standards for making sites that are accessibile to those using assistive technology.
 group: getting-started
 ---
 

docs/getting-started/best-practices.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Best practices
+description: Learn about some of the best practices we've gathered from years of working on and using Bootstrap.
 group: getting-started
 ---
 

docs/getting-started/browsers-devices.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Browsers and devices
+description: Learn which browsers and devices are supported by Bootstrap.
 group: getting-started
 ---
 

docs/getting-started/build-tools.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Build tools
+description: Details on how to use Bootstrap's included build tools to compile source code, run tests, and more.
 group: getting-started
 ---
 

docs/getting-started/contents.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Contents
+description: Learn about what's included in Bootstrap's precompiled and source code directories.
 group: getting-started
 ---
 

docs/getting-started/download.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Download
+description: Download Bootstrap's compiled CSS and JavaScript, source code, or include it with your favorite package manager.
 group: getting-started
 ---
 

docs/getting-started/flexbox.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Flexbox
+description: Learn how to enable flexbox support in Bootstrap 4 with the flick of a variable or the swap of a stylesheet.
 group: getting-started
 ---
 

docs/getting-started/introduction.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Introduction
+description: Get started with Bootstrap using the Bootstrap CDN and a template starter page.
 group: getting-started
 redirect_from: "/getting-started/"
 ---

docs/getting-started/javascript.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: JavaScript
+description: Learn about Bootstrap's JavaScript—how to include it, our data and programmatic API options, and more.
 group: getting-started
 ---
 

docs/getting-started/options.md

@@ -1,6 +1,7 @@
 ---
 layout: docs
 title: Customization options
+description: Customize Bootstrap with Sass variables, easily toggling global preferences with a quick recompile.
 group: getting-started
 ---