docs : add plugin development walkthrough with example plugin

[kavin553]

Description

This PR adds a dedicated plugin development walkthrough to help new contributors create and validate SecuScan plugins more easily.

Changes Made

• Added "docs/plugins/plugin-development-walkthrough.md"
• Included a step-by-step plugin creation guide
• Added example "metadata.json" and "parser.py"
• Documented plugin validation and checksum commands
• Added references to the walkthrough from "README.md" and "PLUGINS.md"

Related Issues

Closes #1044

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Reviewed the documentation flow from plugin creation to validation.
• Verified that all referenced commands already exist in the repository.
• Checked links and references added to "README.md" and "PLUGINS.md".

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[kavin553]

Hi @utksh1 👋

I’ve completed the requested documentation update and opened a PR for review.

Included in this contribution

✅ Added a dedicated plugin development walkthrough

✅ Added example "metadata.json" and "parser.py"

✅ Documented validation and checksum workflows

✅ Added references from "README.md" and "PLUGINS.md"

This guide is intended to make onboarding easier for new contributors and GSSoC participants interested in developing SecuScan plugins.

Thank you for maintaining this project and for the opportunity to contribute. Looking forward to your feedback and review. 🚀

Welcome to any suggestions or improvements!