Add links to IOS and Android versions on web version

[chrismclarke]

Would be good to detect platform, and if web include links to native versions.

Links: Download for Android from Google Play, iOS on App Store, or Use Online

Could be good also to include some form of icon. e.g.

• https://play.google.com/intl/en_us/badges/
• https://www.stickpng.com/img/icons-logos-emojis/store-badges/download-on-the-app-store-badge

[chrismclarke]

Alternatively/additionaly, could use firebase dynamic links as gateway to deeplinks: https://firebase.google.com/docs/dynamic-links/use-cases/web-to-app