docs: better showcase supported dependency managers

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added or modified a feature, documentation in docs is updated

Description of changes

With the recent implementation of PEP 735 dependency groups (and the upcoming setuptools dynamic dependencies one), the dependencies extraction documentation became even more cluttered that it was.

Having a dedicated documentation that lists supported dependency managers would make things way more readable for users, and makes it possible to provide clear examples that explain how deptry extracts dependencies for each manager.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.4%. Comparing base (1210c0d) to head (77e884b).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #895   +/-   ##
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  Coverage   92.4%   92.4%           
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  Files         37      37           
  Lines        994     994           
  Branches      96      96           
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  Hits         919     919           
  Misses        60      60           
  Partials      15      15           

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[mkniewallner]

This section depends on #894 being merged. If it takes more time than expected to get it merged, we can remove the section entirely in this PR.