upgrade eleventy and other dependencies, migrate to ESM

[gaelsimon]

Issue

Describe your changes

• Eleventy v2→v3 and Vite v5→v6 upgrade
  Migrated Eleventy configuration from CommonJS to ESM
  Updated Eleventy from v2 to v3
  Updated Vite from v5 to v6
  Fixed SCSS mixins compatibility
• ESLint v9 flat config migration + cleanup
  ESLint v9: Migrated from .eslintrc.json to eslint.config.mjs using flat config with FlatCompat
  Formatting fix: Restored blank line spacing in transpiled JS output
  TypeScript transpilation was removing blank lines between declarations and functions
  Implemented comprehensive padding-line-between-statements rules for better code readability in documentation
  Decoupled Prettier from ESLint (runs separately now)
  Dependency cleanup: Removed unused packages

How to test

Checklist:

• My code follows the style guidelines for this repo
• My code passes the SonarCloud check and does not add new code smells
• I have performed a self-review of my code
• My changes generate no new warnings/errors
• I have added tests that prove my fix is effective or that my feature works
• I don't require ops changes for this PR to go to prod
• This change does not include a migration

Documentation

Libs/SDKs