Visualization of the nipype codebase

[ivanmilevtues]

Summary

This PR introduces a high-level architectural overview of the Nipype codebase using Mermaid diagrams. These diagrams aim to help new contributors—particularly those from neuroscience and research backgrounds—more easily understand the structure and flow of the code.

You can preview how these diagrams render in another project:
https://github.com/CodeBoarding/GeneratedOnBoardings/blob/main/nipype/on_boarding.md

We’re curious whether this could be helpful for your contributor community and how it might fit into your current onboarding process.

Note: We generate these diagrams using static analysis and LLMs. We're also developing a free GitHub Action that can keep them automatically updated on each merge to main or other branches.

This is part of an early-stage startup exploration focused on improving onboarding for open-source and research-focused projects.

Fixes # .

List of changes proposed in this PR (pull-request)

[x] Added Mermaid-based architectural diagram of the codebase