Remove DOCUMENTATION.md from client repository

[CrystalPea]

It now lives in notifications-tech-docs repository, along with other client docs.

What problem does the pull request solve?

Checklist

• [x] I’ve used the pull request template
• [ ] I’ve written unit tests for these changes
• [ ] I’ve update the documentation (in DOCUMENATION.md and CHANGELOG.md)
• [ ] I’ve bumped the version number (in src/GovukNotify/GovukNotify.csproj)