Added file/folder path mapping to readme

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

Description:

This adds details of how BookStack files and folders within the container /config volume relate to those within a default BookStack installation.

Benefits of this PR and context:

Related to #167.

The default file/folder names used within the mounted /config folder only match those within a BookStack installation by name, and they can be tricky to understand without understanding how these are linked within the container. Users can often need to know/use these files and directories when following BookStack documentation, for things like migrations from non-docker installs, or when switching between image storage options.

How Has This Been Tested?

I have read-through my changes a couple of times, and ran them through the jenkins-builder process then checked the README.md file output to ensure it was valid.

Source / References:

https://github.com/linuxserver/docker-bookstack/issues/167
https://github.com/BookStackApp/BookStack/issues/4161

linuxserver / docker-bookstack