Add doc howto: Recursive workflows with Graph Builder

[agoscinski]

The new doc how-to explains how to create a recursive workflow with the graph builder.

@superstar54 could you explain the mechanic why we need to import the function from a module?

I started to add like a short section at the start of the how-to referring to concepts the user should first familiarize him/herself.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.62%. Comparing base (5937b88) to head (9d5eddb).
Report is 81 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #334      +/-   ##
==========================================
+ Coverage   75.75%   80.62%   +4.87%     
==========================================
  Files          70       66       -4     
  Lines        4615     5147     +532     
==========================================
+ Hits         3496     4150     +654     
+ Misses       1119      997     -122     

Flag	Coverage Δ
python-3.11	80.55% <ø> (+4.88%)	⬆️
python-3.12	80.55% <ø> (?)
python-3.9	80.58% <ø> (+4.84%)	⬆️

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