docs: add CTFd setup guide and update k8s documentation

[seershan]

What kind of changes does this PR include?

• Additional documentation

Summary

This PR adds a dedicated CTFd setup guide and updates the Kubernetes documentation.

Changes made

• Added guides/ctfd/ctfd.md

  • Automated CTFd setup overview
  • Manual CTFd setup instructions
  • Challenge generation and import process
  • 2-domain configuration notes
  • Version compatibility guidance
  • Troubleshooting section

• Updated guides/k8s/k8s.md

  • Removed the outdated warning that referenced Issue Create documentation #79
  • Added a reference to the new CTFd setup guide

Checklist

• All the contributions made are solely the work of me and my co-authors
• I tested the changes in this PR (if applicable)
• I added tests to ensure my change works (if applicable)
• The PR passes pre-commit hooks and automated tests

Related Issue

Partially addresses #79