Extract features of the default template into own packages

[mikebarkmin]

We are currently providing one default template and the ability to specify a custom one. But many features of hyperbook are currently implemented in the default template, thus if one wants to develop their own template most of the features have to be reimplemented or copy and pasted. This might lead to incompatibility issues in the future. Therefore, I propose to extract most of the components and nextjs functions into their own packages, which could be reused in custom templates. Ideally a template just needs to create a custom css and a shell component (header, nav, main).