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

_config.yml

@@ -14,7 +14,7 @@ destination:      _gh_pages
 host:             0.0.0.0
 port:             9001
 baseurl:          ""
-url:              http://getbootstrap.com
+url:              http://v4-alpha.getbootstrap.com
 encoding:         UTF-8
 exclude:          [assets/scss/]
 
@@ -22,6 +22,14 @@ gems:
   - jekyll-redirect-from
   - jekyll-sitemap
 
+# Social
+title: Bootstrap
+description: The most popular HTML, CSS, and JS framework in the world.
+twitter: getbootstrap
+authors: Mark Otto, Jacob Thornton, and Bootstrap contributors
+social_logo_path: /assets/brand/bootstrap-social-logo.png
+social_image_path: /assets/brand/bootstrap-social.png
+
 # Custom vars
 current_version:  4.0.0-alpha.4
 repo:             https://github.com/twbs/bootstrap