Add linux and nmap free tutorials

[sjtushi]

Description

Please include a summary of the resource you're suggesting below:

I work at LabEx, a unique learning platform that has delivered thousands of free and paid hands-on labs over the past 7 years.

• The Linux Commands Practice Course is a free course with 41 labs, covering the most commonly used Linux commands.
• The Nmap Tutorial is a series of free tutorials designed to help learners quickly get started with the basic uses of Nmap.

This repo is a great resource. Please let me know if any changes are needed.

Checklist

Please make sure you reviewed the checklist and comply with each requirement:

• [x] My code follows the contribution guidelines of this project
• [x] This resource is free and focuses on learn by doing style of learning
• [x] This pull request has a title in the format Add Name of Resource:
  • ✅ Add ctf-practice
  • ❌ Update readme.md

⚠️ PLEASE NOTE - Do not expect a prompt review for your PR unless you have truthfully went over contribution guidelines, filled the PR description correctly AND most importantly your changes are passing linters in GitHub Actions pipeline.