Add tutorial for OS (#3015)

* Add link to OS tutorial

* Add introduction

* Update content/roadmaps/101-backend/content/102-os-general-knowledge/readme.md

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

content/roadmaps/101-backend/content/102-os-general-knowledge/readme.md

@@ -4,5 +4,6 @@ Operating System is a program that manages a computer’s resources, especially
 
 <ResourceGroupTitle>Free Content</ResourceGroupTitle>
 <BadgeLink colorScheme='yellow' badgeText='Read' href='https://edu.gcfglobal.org/en/computerbasics/understanding-operating-systems/1/'>What is an operating system?</BadgeLink>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://www.guru99.com/os-tutorial.html'>Operating System summary</BadgeLink>
 <BadgeLink badgeText='Watch' href='https://www.youtube.com/watch?v=26QPDBe-NB8&ab_channel=CrashCourse'>Operating Systems: Crash Course Computer Science #18</BadgeLink>
 <BadgeLink badgeText='Watch' href='https://www.youtube.com/watch?v=vBURTt97EkA&list=PL9hkZBQk8d1zEGbY7ShWCZ2n1gtxqkRrS&index=1'>Introduction to Operating System</BadgeLink>