docs: update core README

[stramel]

Update the README.md file to better explain Icon Packs, styling, and the icon fallback.

Closes #40 Closes #53 Closes #84

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Latest commit: fbdb45e562bc9bb2605363d93bca14a2469ccce0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

| Name | Type | | ---------- | ----- | | astro-icon | Patch | | www | Patch |

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Name	Status	Preview	Comments	Updated (UTC)
api-astroicon	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 25, 2023 5:04pm
astroicon	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 25, 2023 5:04pm