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

Leaflet/Leaflet,Leaflet/Leaflet,msiadak/Leaflet,CapeSepias/Leaflet,CapeSepias/Leaflet,stefanocudini/Leaflet,CapeSepias/Leaflet,stefanocudini/Leaflet,msiadak/Leaflet,stefanocudini/Leaflet,Leaflet/Leaflet,msiadak/Leaflet