docs: add French translation of README

[FelineFantasy]

Base branch: open feature and fix PRs against develop.
Milestone releases promote develop → main — see docs/RELEASE.md.

Summary

Adds French translation of the main README.md file (README.fr.md).

Closes #213

Changes:

• Created README.fr.md with full French translation
• Updated README.md to link to French translation
• Updated README.de.md and README.es.md to link to French translation
• Consistent formatting with existing translations

How to test

1. Verify README.fr.md exists and renders correctly on GitHub
2. Check that all links in README.fr.md point to existing files
3. Verify language flags are present and links work in all README files:
   • README.md (English)
   • README.de.md (German)
   • README.es.md (Spanish)
   • README.fr.md (French)

Code review (Bugbot)

Paused: GitHub bugbot-review is currently a no-op. /review-bugbot is optional.

• N/A — documentation-only change

Checklist

• Documentation updated
• No breaking changes
• Formatting consistent with original

Security / privacy

N/A — Documentation-only change.