docs: update readme to use v6 and switch unpkg to jsdelivr

[ayuhito]

unpkg is unmaintained and full of issues. I switched the documentation over to jsDelivr since it's actively maintained and is much more reliable due to its fallback system. Other CDNs are dependent on Cloudflare only, which wasn't ideal that one time CF went down globally for a couple hours 😅

Also used the broader @6 version tag instead of a specific tag so you don't need to manually change the version until next major release, which you can't really do on unpkg since their implementation adds a redirect hurting performance.

I'll also be making a PR on the website shortly so they can be in sync with this one. PR made: https://github.com/ionic-team/ionicons-site/pull/27