Add required initializers to README and DocC examples

[dfed]

Summary

• add the required public initializers to the README and top-level DocC starter examples
• keep the overview focused on the root and first child instead of expanding into deeper dependency definitions
• simplify the initializer bodies so the examples stay focused on dependency shape rather than implementation detail

Why

• the starter examples should match SafeDI's actual requirements for @Instantiable types
• newcomers should be able to see which initializers are required without extra downstream detail competing for attention

Validation

• docs-only change; not run

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f5be952) to head (3148c51).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main      #285     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           41        41             
  Lines         6940      8419   +1479     
===========================================
+ Hits          6940      8419   +1479     

see 40 files with indirect coverage changes

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