Docs: Improve setup instructions

[RenWal]

It's currently not entirely clear what all the symlinks are that you're supposed to create.

The documentation was my main obstacle as newbie in the whole projects here. I did the same way as worked in fritzbox-munin/Tafkas (unclear too) and made sym.links to FritzboxConfig.py but two times, one as FritzboxConfig.py and the second one FritzboxConfig only.

Originally posted by @Ldwg2002 in #5 (comment)

We should also consider reorganizing the code base into plugins and helper files stored separately from each other to avoid this confusion in the first place.