[INSTALL-6] cmake and ninja-build must be listed as prerequisites

## Problem

After resolving the `CUDA_HOME`/`CPATH` issue, the Transformer Engine build fails because `cmake` is not installed. `ninja-build` is also required but not listed as a prerequisite anywhere in the docs.

## Suggested fix

Add `cmake` and `ninja-build` to the prerequisites list in the installation guide. Consider adding an upfront `apt-get` command that installs all required system packages.