Specify additional steps to utilize GPU for Linux users

Advice to skip additional step 6 if using CPU.

Added second option to create virtual env via Python's built in venv module for Linux users with CUDA-enabled GPUs

Added virtual envs activation/deactivation commands and changed wording for editing the deactivate block in the activate script of the venv virtual env.

Added instructions to resolve the ptxas issue.

Revised CUDNN_DIR definition

Corrected LD_LIBRARY_PATH definition in conda environment instructions

Rename environment variable to PTXAS_DIR and package manager options.

Added note to use pip instead of conda to install TensorFlow.

Removed option to install within conda virtual environment. Recommendation to install in venv environment.

Replaced instructions to modify default activate/deactivate scripts with instructions to create symlinks to NVIDIA shared libraries and ptxas.

Revised the step with instructions to configure the virtual environment variables for GPU users by adding a disclaimer.

Deleted asterisk emoji and placed disclaimer note before respective instructions.