Normalize all links. (#24109)

* use a trailing slash when possible
* use https when possible
* remove a few redirected links
* consistently use `https://popper.js.org/`
* fix `iconUrl` in nuget files
* change Jekyll Windows guide to the official one

scss/mixins/_screen-reader.scss

@@ -1,7 +1,7 @@
 // Only display content to screen readers
 //
-// See: http://a11yproject.com/posts/how-to-hide-content
-// See: http://hugogiraudel.com/2016/10/13/css-hide-and-seek/
+// See: http://a11yproject.com/posts/how-to-hide-content/
+// See: https://hugogiraudel.com/2016/10/13/css-hide-and-seek/
 
 @mixin sr-only {
   position: absolute;