docs(readme): modernize README structure and link wiki setup guides

[Wachhund]

Purpose

• Documentation update
• Bug fix
• New feature
• Refactor/maintenance
• Security hardening
• Other (explain below)

Related Features / Issues

• PROJ: n/a
• Issue: n/a

Overview of Changes

• Reworked README.md into a cleaner, reviewer-friendly structure.
• Kept core project context, but tightened wording and organization.
• Added/kept quick-start installation commands in a compact shell block.
• Added documentation/support section with repo, issues, and discussions links.
• Added practical troubleshooting section for common setup/runtime issues.
• Replaced old local setup-doc reference with wiki-based setup docs:
  • Server Setup Guide (EN)
  • Server Setup Guide (DE)
• Preserved existing mail namespace migration notes.

QA / Verification

• composer run ci:lint: not run (docs-only change)
• composer run test: not run (docs-only change)
• composer run analyse: not run (docs-only change)
• composer run cs-check: not run (docs-only change)
• Manual checks:
  • Verified markdown renders correctly on GitHub preview.
  • Verified both wiki links resolve.

Deployment Impact

• No deploy impact
• Requires deploy runbook update
• Requires environment/config changes
• Requires migration
• Rollback notes: Revert README commit if needed.

Reviewer Focus

• Validate wording/tone for public-facing project intro.
• Confirm wiki links and quick-start instructions match current project reality.

Checklist

• Scope matches linked PROJ / issue
• No secrets or credentials committed
• Docs updated where needed
• Backward compatibility considered
• Ready to merge

[Copilot]

Pull request overview

This PR modernizes the README.md structure to improve readability and usability for new users and contributors. It reorganizes the existing content into clearer sections, adds practical troubleshooting guidance, and redirects detailed setup instructions to wiki-based guides.

Changes:

• Restructured README with improved organization (What Is Included, Requirements table, Installation, Updating, Documentation and Support, Contributing, Troubleshooting, Mail Namespace Migration Notes)
• Replaced non-existent local setup doc reference with wiki-based setup guides (EN and DE)
• Added troubleshooting section covering common setup/runtime issues