feat: Add Markdown Section Templates

[eli64s]

This PR introduces new Markdown section templates for README generation, improving customization options. Key additions include:

• Add CLI Option --header-style: Allows selection of README header style. Three templates currently available.
• Add CLI Option --toc-style: Enables customization of the Table of Contents style. Four templates currently available.
• Additional codebase restructuring and clean up to prepare for more updates coming soon.

This PR marks the beginning of significantly enhancing the customization of the README generation process!

Cheers,
Eli

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Codecov Report

Attention: Patch coverage is 84.18367% with 93 lines in your changes missing coverage. Please review.

Project coverage is 86.23%. Comparing base (a886127) to head (2f6a702).
Report is 1 commits behind head on main.

Files	Patch %	Lines
readmeai/config/settings.py	81.67%	21 Missing and 3 partials ⚠️
readmeai/vcs/url_builder.py	76.08%	10 Missing and 1 partial ⚠️
readmeai/vcs/ingestor.py	83.72%	4 Missing and 3 partials ⚠️
readmeai/__main__.py	84.37%	5 Missing ⚠️
readmeai/models/dalle.py	73.68%	4 Missing and 1 partial ⚠️
readmeai/templates/header.py	80.00%	4 Missing ⚠️
readmeai/templates/toc.py	90.00%	3 Missing and 1 partial ⚠️
readmeai/vcs/errors.py	75.00%	4 Missing ⚠️
readmeai/models/openai.py	78.57%	2 Missing and 1 partial ⚠️
readmeai/parsers/cpp.py	50.00%	3 Missing ⚠️
... and 17 more

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   91.94%   86.23%   -5.71%     
==========================================
  Files          39       43       +4     
  Lines        1675     1664      -11     
  Branches        0      264     +264     
==========================================
- Hits         1540     1435     -105     
- Misses        135      169      +34     
- Partials        0       60      +60     

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