extract the theme as a standalone package

[moebiusmania]

The current theme is a customization of the ejected default one. By now it covers our needs, but in the future we may want to radically redesign the website while keeping the content as-is or we may want to use this one on different VuePress websites without manually hard-encoding it.

Following the official documentation:
https://vuepress.vuejs.org/theme/using-a-theme.html#using-a-theme-from-a-dependency

We need to:

• create a new repository amber-vuepress-theme
• move the related code there
• publish it on NPM as @amber-ds/vuepress-theme-docs
• add it as a dependency here