Add-app-manifest

[markhallen]

• Add docs/app-manifest.json with complete Slack app configuration
• Update README with manifest-based setup as recommended option
• Remove sensitive URLs from manifest (use placeholder domains)
• Restructure Slack setup documentation with two clear options:
  • Option 1: Create from manifest (recommended)
  • Option 2: Manual setup (existing approach)
• Add manifest reference to project structure documentation
• Improve Slack app configuration section with clearer guidance

This makes it much easier for users to create their own Slack app
by simply copying the manifest and updating the domain URLs.

Description

Brief description of the changes in this PR.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring/code cleanup

Testing

• Tests pass locally
• New tests added for new functionality
• Manual testing completed

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

Add any additional notes or context about the PR here.