Add meta information to each page

[OliverDudgeon]

We should

• [x] Set titles on each main page using next/head Head component
• [ ] Page descriptions via Head
• [x] Fix the favicon not loading from the root site
• [ ] Other similar SEO optimisations to be added

[OliverDudgeon]

Note that after switching to xdm, the next/head Head component is required in mdx files

[OliverDudgeon]

The main squonk site has been updated which the dm-ui sources it's icon from