feat(markdown): prioritize README.md files over _index.md files

[monotykamary]

Summary

• Modified export_markdown.ex to prioritize README.md files over _index.md files
• When a README.md file exists in a directory, it will be converted to _index.md in the export path
• If both README.md and _index.md exist in the same directory, README.md is used and _index.md is ignored

Technical Implementation

• Updated the file processing logic to handle the README.md → _index.md conversion as part of the export process
• Modified the directory processing to filter out _index.md files when a README.md exists in the same directory
• Fixed the slugification process to ensure that converted _index.md files are not slugified to lowercase
• Ensured that frontmatter validation is still applied to all files including README.md files

Test Plan

• Verify that a README.md file in a directory is properly converted to _index.md in the export path
• Confirm that when both files exist, README.md content appears in the _index.md output
• Check that the proper Hugo compilation occurs with the converted files

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