Added "path_prefix" to default config

[x] I have read the contributing guideline and understand that I have made the correct modifications

Description:

With the current default config, path_prefix is undefined, which breaks the "edit" button on pages. Clicking it will redirect to a dead link. also added more verbose explanations of this and the "base_url" value from the master repo.

Benefits of this PR and context:

This fixes the issue that the edit button on pages does not point to the correct url when using the default config. With the value undefined, clicking the edit button will direct to <basedomain.tld>undefined/<subpath>. Adding the empty definition fixes this issue.

It's a very simple change but will help less experienced people who won't dig through the source code to find this solution.

How Has This Been Tested?

I added it and now the button works.

Source / References:

Original config.js which has this value: https://github.com/ryanlelek/Raneto/blob/master/config/config.js

linuxserver / docker-raneto