Make sidebar easy to navigate and refactor config file

This PR will make the sidebar easy to navigate and it also refactors the config file by extracting some fields into their own files. I think the refactoring is necessary because such config files can get out of hand pretty quickly if all configuration options are kept in the same config file.

The main goal of this PR is to make the top-level sidebar items collapsible like the references section of the vuePress documentation. However, the collapsible sidebar feature was added in version 2.0.0-beta.31 and we are still using 2.0.0-beta.27.

Let me know if you think this will ease sidebar navigation so that I can also refactor the extensions section in a similar way.

Fixes #39 (closed)