[docs] Visualizing the dgl codebase

## Description <!-- Brief description. Refer to the related issues if existed. It'll be great if relevant reviewers can be assigned as well.--> This PR gives a diagram of your python code (Static analysis + LLMs). You can see how it renders here: https://github.com/CodeBoarding/GeneratedOnBoardings/blob/main/dgl/on_boarding.md The idea is to give high-level udnerstanding of contributors when they are starting on how the whole system works. So that they can have context when starting to work on their first issue - knowing how the small piece fits in the big picture! I saw you have a docs pipelien if you like what you see I'd love to integrate it for you (also with our free github action to keep the docs up-to-date). I usually open discussions, but they are not enabled for your repo. Any feedback is more than welcome! Full transparency: we’re exploring this idea as a potential startup, but we’re still early and figuring out what’s actually useful to developers. ## Checklist Please feel free to remove inapplicable items for your PR. - [x] The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]]) - [x] The PR is complete and small, read the [Google eng practice (CL equals to PR)](https://google.github.io/eng-practices/review/developer/small-cls.html) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted). ## Changes Added docs :)