docs: Switch to StackPath's URL.

While the old one will keep working, better switch to the new one.

We made the switch on BootstrapCDN just yesterday, everything is looking good, so it's a good time to change the references here too.

/CC @jdorfman

docs/4.0/getting-started/introduction.md

@@ -13,7 +13,7 @@ toc: true
 
 ## Quick start
 
-Looking to quickly add Bootstrap to your project? Use BootstrapCDN, provided for free by the folks at MaxCDN. Using a package manager or need to download the source files? [Head to the downloads page.]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/download/)
+Looking to quickly add Bootstrap to your project? Use BootstrapCDN, provided for free by the folks at StackPath. Using a package manager or need to download the source files? [Head to the downloads page.]({{ site.baseurl }}/docs/{{ site.docs_version }}/getting-started/download/)
 
 ### CSS
 

_config.yml

@@ -67,9 +67,9 @@ download:
 
 cdn:
   # See https://www.srihash.org for info on how to generate the hashes
-  css:              "https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css"
+  css:              "https://stackpath.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css"
   css_hash:         "sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm"
-  js:               "https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"
+  js:               "https://stackpath.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"
   js_hash:          "sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl"
   jquery:           "https://code.jquery.com/jquery-3.3.1.slim.min.js"
   jquery_hash:      "sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo"