refactor(BaseConfig): update docstring, extract factory method and remove unnecessary variable assignment

[bearomorphism]

Description

Checklist

• I have read the contributing guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run poetry all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run poetry doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external) in the documentation

When running poetry doc, any broken internal documentation links will be reported in the console output like this:

INFO    -  Doc file 'config.md' contains a link 'commands/bump.md#-post_bump_hooks', but the doc 'commands/bump.md' does not contain an anchor '#-post_bump_hooks'.

Expected Behavior

Steps to Test This Pull Request

Additional Context

[codecov]

Codecov Report

❌ Patch coverage is 97.05882% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (v4-9-2@a472bea). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
commitizen/config/__init__.py	95.65%	1 Missing ⚠️
commitizen/config/factory.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             v4-9-2    #1601   +/-   ##
=========================================
  Coverage          ?   98.25%           
=========================================
  Files             ?       59           
  Lines             ?     2690           
  Branches          ?        0           
=========================================
  Hits              ?     2643           
  Misses            ?       47           
  Partials          ?        0           

Flag	Coverage Δ
unittests	98.25% <97.05%> (?)

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