Add documentation for nuxt config

[bnznamco]

In the latest realeases (39-40-..) we added some configuration options for nuxt config.
For example:

 {
      content: {
          languages: ['it', 'en']
      },
      integrations: {
          camomilla: {
              location: "@mapomodule/mapo-integrations-camomilla",
              configuration: {
                  api: {
                      target: "http://localhost:8000",
                      syncCamomillaSession: true,
                    headers: {
                        "x-Forwarded-Host": "localhost",
                    },
                },
            },
        },
    },
    ui: {
        panelName: "MAPO PANEL",
        // panelLogo: "https://cdn.icon-icons.com/icons2/1954/PNG/512/orangefruit_122731.png",
        forms: {
            fields: {
                attrs: {
                    vSwitch: { color: "secondary" },
                }
            },
            groups: {
                expanded: true
            }
        },
        sidebar: {
            spiderMenu: true
        }
    }
}

We need to document all the possible configurations.