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/http.md

@@ -8,7 +8,7 @@
 
 - Send form-encoded data:
 
-`http -f {{example.org}} {{name='bob'}} {{profile-picture@'bob.png'}}`
+`http -f {{example.org}} {{name='bob'}} {{profile_picture@'bob.png'}}`
 
 - Send JSON object: