ci: stop dependabot from scanning the repository root for pip

[mwbrooks]

Type of change

• New feature
• Bug fix
• Documentation

Summary

This pull request stops Dependabot from scanning the repository root for pip updates, so maintainers get one bump PR per dependency instead of two that could drift apart.

• Removes "/" from the pip directories list in .github/dependabot.yml
• The root has no dependency file. That entry only ever bumped the requirements.txt files in each subfolder, which were removed in chore: remove requirements.txt in favour of pyproject.toml #247
• Each subfolder entry keeps watching its own pyproject.toml

Requirements

• I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• I've read and agree to the Code of Conduct