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/parallel.md

@@ -20,7 +20,7 @@
 
 - Break stdin into ~1M blocks, feed each block to stdin of new command:
 
-`cat {{bigfile.txt}} | parallel --pipe --block 1M {{command}}`
+`cat {{big_file.txt}} | parallel --pipe --block 1M {{command}}`
 
 - Run on multiple machines via SSH: