Added .gitkeep placeholders for Days 11–30

As part of building the Bash scripting course, I’ve added `.gitkeep` files to `day11` through `day30` so these folders are tracked in git but I'm updating every day as I learn. 

These days are currently empty and will be filled with scripts and examples as the course progresses.  

This is mainly to set up the structure for the rest of the course while I’m still learning.