docs: expand style guide, add CONTRIBUTING.md, clarify line-length policy

[Copilot]

Overview

This PR expands and improves the Nextcloud documentation contributor guidelines and style standards.

Changes

1. Enhanced Style Guide (style_guide.rst)

• Expanded documentation with clearer formatting and writing standards
• Clarified line-length policy for better consistency across documentation
• Improved PHP code examples with proper context and array demonstrations
• Fixed sudo command syntax: sudo -u www-data php occ → sudo -E -u www-data php occ

2. New Contributor Guide (CONTRIBUTING.md)

• Added comprehensive contributor guidelines for the documentation repository
• Provides clear onboarding for new documentation contributors

3. Communication Updates

• Updated communication channels from Matrix/IRC to Nextcloud Talk
• Added cross-manual linking rules to improve documentation navigation

Why This Matters

• Better contributor experience: Clear guidelines make it easier for people to contribute high-quality documentation
• Consistency: Expanded style guide ensures uniform documentation across the project
• Accessibility: Updated communication channels reflect current Nextcloud community practices
• Quality: Improved examples and clarified syntax reduce errors in documentation

Files Modified

• style_guide.rst - Updated with expanded guidelines, clarified line-length policy, and corrected PHP examples
• CONTRIBUTING.md - New file with comprehensive contributor guidelines
• Communication channel references - Updated to use Nextcloud Talk