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

_config.yml

@@ -54,15 +54,15 @@ current_version:    4.0.0-beta
 docs_version:       4.0
 repo:               "https://github.com/twbs/bootstrap"
 slack:              "https://bootstrap-slack.herokuapp.com"
-download:
-  source:           "https://github.com/twbs/bootstrap/archive/v4.0.0-beta.zip"
-  dist:             "https://github.com/twbs/bootstrap/releases/download/v4.0.0-beta/bootstrap-4.0.0-beta-dist.zip"
-
 blog:               "https://blog.getbootstrap.com"
 expo:               "https://expo.getbootstrap.com"
 jobs:               "https://jobs.getbootstrap.com"
 themes:             "https://themes.getbootstrap.com"
 
+download:
+  source:           "https://github.com/twbs/bootstrap/archive/v4.0.0-beta.zip"
+  dist:             "https://github.com/twbs/bootstrap/releases/download/v4.0.0-beta/bootstrap-4.0.0-beta-dist.zip"
+
 cdn:
   # See https://www.srihash.org for info on how to generate the hashes
   css:              "https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css"