Add clarification that remote fonts are downloaded

[JusticeMatthew]

📚 Subject area/topic

Fonts

📋 Page(s) affected (or suggested, for new content)

https://docs.astro.build/en/reference/experimental-flags/fonts/

📋 Description of content that is out-of-date or incorrect

Add some clarification about how remote fonts are handled, specifically that they are downloaded to the project

This, in my case, actually caused the "remote" fonts to be more performant than when using the same fonts locally. (I presume this is due to them being downloaded to node_modules somewhere)

Additionally, this will clarify to users concerned about GDPR that the fonts are being downloaded and effectively "self-hosted"

🖥️ Reproduction in StackBlitz (if reporting incorrect content or code samples)

No response