Doc modification + info about slashes in filename if using VDFS (#45)

Co-authored-by: Rafał Mastalerz <rafal@liwrama.pl>

docs/docs/user-guide/concepts/index.md

@@ -31,11 +31,7 @@ The area where a song plays. Music zone are created using Spacer and a song may
 
 Virtual file system used by Gothic to load assets. zBassMusic supports it fully, and we may reference some directory as:
 
-### Physical
+* Physical - Located on your computer hard drive in the Gothic installation directory.
 
-Located on your computer hard drive in the Gothic installation directory
-
-### Virtual
-
-A *.vdf or *.mod file loaded by Gothic, they contain the same directory structure as physical, 
+* Virtual - A *.vdf or *.mod file loaded by Gothic, they contain the same directory structure as physical, 
 but all files are inside the file (like a ZIP archive).

docs/docs/user-guide/getting-started/index.md

@@ -37,7 +37,7 @@ Alternatively, below you should see the newest release:
 
 </script>
 
-There are different kinds of files for different use cases.
+There are different kinds of files for different use cases:
 
 * `zBassMusic-{version}.zip`<br>ZIP archive with zBassMusic.dll and its dependencies. Use for loading the plugin
   directly from the disk.

docs/docs/user-guide/theme-definition/basstheme.md

@@ -75,3 +75,6 @@ The audio `theme` field must be the same as theme's `name` to assign it for this
 The audio instance must  use `DEFAULT` as a `type`, in order to play. 
 In the future, we may introduce features that use multiple files per theme, hence this separation.
 
+The `filename` field contains the name of the file that should be played in the custom theme. 
+Note that when using Union, only the filename should be provided, regardless of whether it is located in subdirectories 
+of the Music folder.