docs: remove slugs and update links to md

[danielclow]

1. remove custom slugs
2. update all links to use .md file instead of url
3. update hardcoded links to use .md file
4. lowercase all md filenames

[gemini-code-assist]

Code Review

This pull request performs a comprehensive refactoring of the English documentation, primarily standardizing internal cross-references by replacing directory-based slugs with direct file paths and removing redundant slug metadata from frontmatter. It also updates several external URLs to ensure consistency across the documentation set. A syntax error was identified in a Markdown link within the machine learning documentation where a double opening parenthesis was incorrectly used in a file path.