Expand the fonts guide with variable fonts, preloading & fontData

[ArmandPhilippot]

Description (required)

We had several reports here (#13635, #13583, #13324) and on Discord about the fonts guide:

• using variable fonts is not clear enough
• because we have several pages related to fonts, available props are not easy to find (we had a single link to the <Font /> reference) and some people wants to preload their fonts
• the OG image example wasn't working

Regarding the last point, I know Florian plans to improve the API but in the meantime, I think it's a good idea to improve the documentation, because this specific use case is of interest to many people.

This PR:

• adds a new section for preloading fonts
• adds a new section for using variable fonts, with remote and local fonts
• adds a new section for fallback fonts
• expands the "Accessing font data programmatically" section to:
  • provide a code snippet that can work for both dev and build
  • provide a full working example (~20 lines added...)
  • highlight this is only an example that won't work for everyone (multiple font files, adapter output...)
• expands the FontData type description, I think this can be helpful for people using font data programmatically.

cc @florian-lefebvre (no rush, this is in draft on purpose!), feel free to comment on the changes and/or push to my branch directly if your prefer!
I know you didn't want to highlight preload but since this option is available, I think it makes sense to mention it in the guide, with the same caution.

Related issues & labels (optional)

• Closes Incorrect guide for accessing font programatically through Fonts API #13635
• Partially answer: Documentation missing on how to use variable fonts with new font API #13583
• Suggested label: add new content, improve or update the documentation

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[florian-lefebvre]

Looks awesome! Just one comment

[yanthomasdev]

Amazing work here, two small suggestions from me so far.

[ArmandPhilippot]

Because the suggestions couldn't be applied (I don't know what happened with dbb79ed... The entire file was considered modified), I just pushed three new commits with what we discussed:

• Local provider/Other providers (with local first), including some rewording (5b5e788)
• Preloading fonts as an h2 (8c29de7)
• Reword the Satori code snippet introduction to add a new link... I used my last suggestion, but if you don't like it, we can update it with a two sentences format. (c2c53b7)