Add code coverage visualization for php

What does this MR do and why?

Added section documenting how to do coverage visualization for php projects.

Screenshots or screen recordings

How to set up and validate locally

1. Fork this repository
2. Create a new MR to your local fork.
3. Check out the code coverage visualization in the MR interface.
4. Check the file changes in this MR to see if they match that of the repository in step 1.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.