Use nuxt/content v3

[imphil]

We are currently using nuxt/content v2. Switch to v3 once we can. Currently it seems blocked by nuxt-content-assets not being compatible (davestewart/nuxt-content-assets#90).

Migration docs: https://content.nuxt.com/docs/getting-started/migration

WIP branch: https://github.com/fossi-foundation/fossi-foundation-web/tree/nuxt-content-3

[imphil]

nuxt/content#3140 (comment) might be a workaround if nuxt-content-assets doesn't live on.