Docs(Download): add CDN alternatives (#5837)

* Docs(download): mention other CDN

so that developers know their possible alternatives and can decide by themselves which option they want to use for their project.
None of the 3 listed options require any work from Leaflet side (as of today) for new releases, they directly pull updated from npm registry.
cdnjs uses a configuration file in their repo to know which files to fetch from npm registry:
https://github.com/cdnjs/cdnjs/blob/master/ajax/libs/leaflet/package.json
In case the `dist/` folder changes, this configuration file requires manual modification (the community may directly send a PR to do so).

* Docs(download): add disclaimer about external CDN services

in order to try avoiding questions and/or complaints in case of CDN unavailability or any other issue.

docs/download.md

@@ -32,7 +32,7 @@ so please read the changelog carefully when upgrading to it.
 
 ### Using a Hosted Version of Leaflet
 
-The latest stable Leaflet release is hosted on a CDN — to start using
+The latest stable Leaflet release is available on several CDN's — to start using
 it straight away, place this in the `head` of your HTML code:
 
     <link rel="stylesheet" href="https://unpkg.com/leaflet@{{ site.latest_leaflet_version}}/dist/leaflet.css" />
@@ -49,6 +49,14 @@ when using Leaflet from a CDN:
       integrity="{{site.integrity_hash_uglified}}"
       crossorigin=""></script>
 
+Leaflet is available on the following free CDN's:
+
+- unpkg: https://unpkg.com/leaflet/dist/
+- cdnjs: https://cdnjs.com/libraries/leaflet
+- jsDelivr: https://www.jsdelivr.com/package/npm/leaflet?path=dist
+
+_Disclaimer: these services are external to Leaflet; for questions or support, please contact them directly._
+
 ### Using a Downloaded Version of Leaflet
 
 Inside the archives downloaded from the above links, you will see four things: