feature (theme): add light/dark mode toggle across all mkdocs configs

[AarishShah]

Summary

Adds a light/dark mode toggle to the OWASP Top 10 documentation site by configuring the palette option in the MkDocs Material theme.

Changes

Added a palette block to:

• mkdocs.yml
• 2021/mkdocs.yml
• 2025/mkdocs.yml

Two schemes:

• default: light mode with a ☀️ toggle to switch to dark
• slate: dark mode with a 🌙 toggle to switch to light

No dependencies added.

Reason

The OWASP Top 10 site currently has no dark mode. The MkDocs Material theme
supports this natively with a simple config change which has been applied.

Testing

Tested locally:

• 2021/: python -m mkdocs serve from 2021/ directory, all language builds passed
• 2025/: python -m mkdocs serve from 2025/ directory, English build passed
• Zero new errors introduced