docs/BLS: rework the description of directory layout

docs/BLS: rework the description of directory layout

We said "`$BOOT/loader/` is the directory containing all files needed
for Type #1 entries" which is blatantly wrong. And also saying that we
define two directories, /loader and /loader/entries, but only ever defining
the second one was not very consistent.

Instead, let's say that /loader/ is for "boot loader configuration", and
/loader/entries has the snippets. A new section about /<entry-token>/<version>/
is added. This is described as the "recommended layout for additional files".

Also, we said that ID= should be used in the file name, but in fact it
wasn't in the example that was given, and afaik, nobody ever did that. So
this part is reduced to say "kernel version (as returned by `uname -r`,
including the OS identifier)". AFAIK, all distros include some form of
OS identifier in the version, so this should be good enough.

Since we now don't depend on autodetection (e.g. with entry-token and layout
configured), the installed doesn't need to always create /loader/entries and
things will still work. So don't say that the installer needs to create it.

Part of the discussion is moved to the Discussion section.

Overall, this brings the specification more in line with actual practice.