Improve missing rules file status and messaging

[dkargatzis]

Overview

This PR addresses described in issue #9:

• Misleading "success" status when rules file is missing - addressed by providing clear, actionable feedback when rules are not configured
• Support for testing rules in feature branches - still under consideration

Changes

• Change missing rules file status from "success" to "neutral"
• Add clear setup instructions with examples
• Clarify that rules are read from main branch only
• Update documentation links to correct configuration page

[watchflow]

❌ Watchflow rules file not found

The file .watchflow/rules.yaml is missing from your repository.

How to fix:

1. Create a file at .watchflow/rules.yaml in your repository root.
2. Add your rules in the following format:

   rules:
     - id: example-rule
       description: Example rule description
       ...

3. Read the documentation for more details.

After adding the file, push your changes to re-run validation.