feat: update greeting format to include '@' for user mentions in templates (#27)

update greeting format to include '@' for user mentions in templates

Author: FidelusAleksander

markdown-templates/readme/exercise-started.md

@@ -2,7 +2,7 @@
 
 <img src="https://octodex.github.com/images/Professortocat_v2.png" align="right" height="200px" />
 
-Hey **{{ login }}**!
+Hey @{{ login }}!
 
 Mona here. I'm done preparing your exercise. Hope you enjoy! 💚
 

markdown-templates/step-feedback/exercise-finished.md

@@ -1,7 +1,7 @@
 <img src="https://octodex.github.com/images/welcometocat.png" align="left" height="150px" />
 
-Congratulations **{{ login }}**! You finished the exercise! 🎉🎉🎉
+Congratulations @{{ login }}! You finished the exercise! 🎉🎉🎉
 
 We've updated the repository with a couple changes to highlight your success!
 
-Return to the [repository home](/{{ repo_full_name }}) page to see your progress!
+Return to the [repository home](/{{ repo_full_name }}) page to see your progress!

markdown-templates/step-feedback/welcome.md

@@ -2,7 +2,7 @@
 
 <img alt="original github octocat" src="https://octodex.github.com/images/original.png" align="left" height="80px" />
 
-👋 Hey there {{ login }}! Welcome to your Skills exercise!
+👋 Hey there @{{ login }}! Welcome to your Skills exercise!
 
 {{ intro_message }}