Added a process flow diagram for better explainability

[mohan-the-octocat]

This pull request introduces a significant update to the docs/process-flow.md file. The changes provide a detailed explanation of how the GHAS-Reviewer app operates when developers dismiss alerts on the GitHub UI. It also includes a sequence diagram to visually represent the process flow.

Key changes include:

• Added a new section that outlines the process flow of the GHAS-Reviewer app when developers dismiss alerts on the GitHub UI. This includes the triggering of a webhook, processing of the payload by the GHAS-Reviewer app, and the subsequent actions taken based on the type of alert dismissed.
• Included a sequence diagram to visually represent the interaction between the Developer, GitHub UI, GHAS-Reviewer app, and the Security Team. The diagram helps in understanding the sequence of events that take place when an alert is dismissed by a developer.

[GeekMasher]

Amazing!