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/cat.md

@@ -8,11 +8,11 @@
 
 - Concatenate several files into the target file:
 
-`cat {{file1}} {{file2}} > {{target-file}}`
+`cat {{file1}} {{file2}} > {{target_file}}`
 
 - Append several files into the target file:
 
-`cat {{file1}} {{file2}} >> {{target-file}}`
+`cat {{file1}} {{file2}} >> {{target_file}}`
 
 - Number all output lines: