docs: update readme to use latest 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: ionic-team/ionicons-site#27

[liamdebeasi]

Thanks for the PR and apologies for the delay! I think this is a good change to make. Using jsdelivr aligns with how we do things on Ionic Framework too.

I made a couple changes:

1. Instead of using the @6 tag I just changed it to use @latest that way we don't need to update this every time there's a new major release of Ionicons.
2. I added the unpkg part back to the package.json. We'll want to keep this configuration until we can make a breaking change in a major release of Ionicons. If we removed that config now anyone pulling the @latest tag on unpkg could run into issues.