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/ed.md

@@ -28,7 +28,7 @@
 
 - Write the current document to a new file (the filename can be omitted if `ed` was called with an existing file):
 
-`w {{filename.txt}}`
+`w {{filename}}`
 
 - Quit ed: