Fix "thirdparty" folder

[GPMueller]

Closes #79.

• [x] remove unneeded "thirdparty" folders from folder trees in docs
• [x] remove special treatment of "thirdparty" folder
• [x] fix the Readme accordingly

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Some thoughts about the thirdparty folder:

It would be nice to automatically treat "thirdparty" folders, removing the need for a build config for many projects, if the dependencies inside also adhere to a reasonable folder structure. In any case, this should only be done if no build configuration was given.

This could be done by assuming a folder structure "thirdparty/targetname" etc., but this might be too unstable with respect to minor alterations of the folder structure. More cases could be covered without build config if this were also done for the root folder, which could contain e.g. "myexe" and "mylib" folders corresponding to two respective targets.

[codecov[bot]]

Codecov Report

Merging #95 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #95   +/-   ##
=======================================
  Coverage   80.91%   80.91%           
=======================================
  Files          12       12           
  Lines         985      985           
=======================================
  Hits          797      797           
  Misses        188      188

Impacted Files	Coverage Δ
clang_build/io_tools.py	100% <100%> (ø)	:arrow_up:

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