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/git-diff.md

@@ -20,4 +20,4 @@
 
 - Create a patch file:
 
-`git diff > {{target-file.patch}}`
+`git diff > {{target_file.patch}}`