Improve footer link flexibility

Deprecate hardcoded footer links in favor or new `footer.links` array for any link and in any order. All of Font Awesome's icons are available for use.

```
footer:
  links:
    - label: "Twitter"
      icon: "fab fa-fw fa-twitter-square"
      url: "https://twitter.com/username"
    - label: "GitHub"
      icon: "fab fa-fw fa-github"
      url: "https://github.com/username"
    - label: "Instagram"
      icon: "fab fa-fw fa-instagram"
      url: "https://instagram.com/username"
```

docs/_docs/05-configuration.md

@@ -768,6 +768,35 @@ author:
 
 To customize the author sidebar, read the full [layout documentation]({{ "/docs/layouts/#author-profile" | relative_url }}).
 
+## Site Footer
+
+Footer links can be added under the `footer.links` array.
+
+| Name | Description |
+| --- | --- |
+| **label** | Link label (e.g. `"Twitter"`) |
+| **icon** | [Font Awesome icon](https://fontawesome.com/icons?d=gallery) classes (e.g. `"fab fa-fw fa-twitter-square"`) |
+| **url** | Link URL (e.g. `"https://twitter.com/mmistakes"`) |
+
+```yaml
+footer:
+  links:
+    - label: "Twitter"
+      icon: "fab fa-fw fa-twitter-square"
+      url: "https://twitter.com/mmistakes"
+    - label: "GitHub"
+      icon: "fab fa-fw fa-github"
+      url: "https://github.com/mmistakes"
+    - label: "Instagram"
+      icon: "fab fa-fw fa-instagram"
+      url: "https://instagram.com/mmistakes"
+```
+
+**Note:** Twitter and Facebook footer links no longer automatically pull from `site.twitter.username` and `site.facebook.username`. This behavior has been deprecated in favor of the `footer.links` array above.
+{: .notice--danger}
+
+To change "Follow:" text that precedes footer links, edit the `follow_label` key in `_data/ui-text.yml`.
+
 ## Reading Files
 
 Nothing out of the ordinary here. `include` and `exclude` may be the only things you need to alter.