Updating Documentation for FetchContent Changes

[FlorianXXIV]

After creating targets for CMakes FetchContent in #217 I updated the documentation so that the shown target is correct.
So
from target_link_libraries(${PROJECT_NAME} PRIVATE Graaf_lib)
to target_link_libraries(${PROJECT_NAME} PRIVATE Graaf::Graaf)

I also added a section about the CMake Options that are available, and gave examples on how to use them when people include this library for their Project.

I verified the documentation building locally.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.57%. Comparing base (7bebd60) to head (6a8d969).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #224   +/-   ##
=======================================
  Coverage   99.57%   99.57%           
=======================================
  Files          56       56           
  Lines        2586     2586           
  Branches      135      135           
=======================================
  Hits         2575     2575           
  Misses         11       11           

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[bobluppes]

Thanks a lot for improving the documentation here!
LGTM