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/common/atom.md

@@ -5,8 +5,8 @@
 
 - Open a file or folder:
 
-`atom {{path/to/file/or/folder}}`
+`atom {{path/to/file_or_folder}}`
 
 - Open a file or folder in a new window:
 
-`atom -n {{path/to/file/or/folder}}`
+`atom -n {{path/to/file_or_folder}}`