[docs-infra] Move to `next/font`

[Janpot]

Use built-in Next.js font optimization over fg-loadcss.

Advantages:

• Performance: TODO Measure potential impact on FCP (positive or negative?)
• Security: Get rid of SRI warnings for google fonts in securityscorecard

[dav-is]

@Janpot Security: Get rid of SRI warnings for google fonts in security scanner

Could you link to the scanner?