Mention about Portable Computing Language (PoCL) to help in development

Hii.

Currently the [Getting Started Guide for Linux](https://github.com/KhronosGroup/OpenCL-Guide/blob/main/chapters/getting_started_linux.md) mentions about installing the headers to develop the applications. But if they want to run the application, then they need an OpenCL implementation.

We have `pocl-opencl-icd` in Debian and other Linux distributions and similar tools for other operating systems. The guide can be updated to include details about `PoCL` too so users can use that as an entry-point to run and test their code.

Start with chapters/getting_started_linux.md and review how it currently describes installing headers for OpenCL development. Add information about PoCL, including the pocl-opencl-icd package mentioned for Debian and other Linux distributions, so readers can run and test applications. Done means the guide explains this implementation entry point without removing the existing setup guidance.

Written by the indexing model from the issue text.