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/linux/pkgadd.md

@@ -4,8 +4,8 @@
 
 - Install a local software package:
 
-`pkgadd {{package-name}}`
+`pkgadd {{package_name}}`
 
 - Update an already installed package from a local package:
 
-`pkgadd -u {{package-name}}`
+`pkgadd -u {{package_name}}`