chore: Fix markdown linting violations in README templates

[actuarysailor]

Summary

This PR resolves markdown linting violations across all markdown files in the repository to ensure consistent, high-quality markdown formatting for the README template project.

Changes Made

• Fixed MD041: Added proper top-level headings where missing
• Fixed MD033: Added blank lines around HTML blocks for better structure
• Fixed MD013: Wrapped long lines to stay under 80 characters where practical
• Fixed MD034: Wrapped bare URLs in angle brackets (<>)
• Fixed MD032: Added proper spacing around lists
• Fixed MD010: Standardized list formatting (hyphens instead of asterisks)
• Fixed MD047: Added blank line at end of files
• Character cleanup: Replaced HTML entities with cleaner markdown equivalents

Files Updated (6 total)

• README.md - Main template with example content
• BLANK_README.md - Clean template for users to customize
• CHANGELOG.md - Project changelog formatting
• .github/ISSUE_TEMPLATE/*.md - Issue template files
• .markdownlint.json - Added linting configuration (new file)

Testing

• All markdown files pass linting with the new .markdownlint.json configuration
• Templates maintain visual appeal and functionality
• HTML elements required for logos and layout are properly configured as exceptions
• Issue templates render correctly

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Documentation update
• Code quality improvement

This change improves template quality and maintainability while preserving backward compatibility and visual presentation.