Librarian documentation

[Cortys]

As discussed, I now added more detailed documentation for the code I worked on. There are no changes to the implementation.

This PR includes:

• Added docstrings to all non-trivial functions to make the code easier to understand.
• Added usage and architecture guides to the modules that make up the librarian toolkit.
• Added a short getting started guide for development environment setup, where to start with the documentation and so on.
• Added a future tasks section with a prioritized list of things that could be worked on next.

The rendered documentation files can be read here.

@adibaba @bigerl Let me know if you think some parts should be explained in more detail.

[codecov-io]

Codecov Report

Merging #171 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #171   +/-   ##
=========================================
  Coverage     43.32%   43.32%           
  Complexity      170      170           
=========================================
  Files            51       51           
  Lines          1311     1311           
  Branches        141      141           
=========================================
  Hits            568      568           
  Misses          707      707           
  Partials         36       36

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