Applying the snake_case convention throughout the repo (#967)

* Applying the snake_case convention throughout the repo

- Also removing the file extension where not needed
- Adding {{ }} on a few old pages

* Addressing concerns

- Added {{ }} where they were missed out
- Removed spaces inside {{ }}
- Reverting "file" to "filename" to make it clearer

* Fixing the comments on nc page

pages/linux/htop.md

@@ -8,7 +8,7 @@
 
 - Start htop displaying only processes owned by given user:
 
-`htop -u {{user-name}}`
+`htop -u {{user_name}}`
 
 - Get help about interactive commands: