docs: assets (pubic and processed)

[jbmoelker]

Important

Note: #209 should be merged first.

Changes

• Adds documentation for assets (both public and processed), including custom fonts and icons.
• Moves documentation on icons from Icon component to new assets documentation.
• Links to the new assets documentation from all relevant directories.

Associated issue

Resolves #210

How to test

• Review the new documentation.
• Verify that the described procedures work.

Checklist

• I have performed a self-review of my own code
• I have made sure that my PR is easy to review (not too big, includes comments)
• I have made updated relevant documentation files (in project README, docs/, etc)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• I have notified a reviewer

[cloudflare-workers-and-pages]

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