docs: adding information to CONTRIBUTING.md that *.d.ts files are generated based on markdown (md) files

[dev-jonghoonpark]

Hello, I recently submitted my first pull request (#28231) regarding toPass timeout configuration.

When I first saw this project, I didn't anticipate that *.d.ts files would be generated through markdown(md) files. As a result, it took me more time to analyze the code.

At the top of the test.d.ts file, it is mentioned:
// This file is generated by /utils/generate_types/index.js
However, since the concept of generating *.d.ts files through md files was new to me, I hadn't considered that at all.

Adding information to the contributing document that *.d.ts files are generated based on md files could be helpful for beginners like me.

For example,

All API classes, methods, and events should have a description in docs/src.
Type declaration files (e.g. test.d.ts, testReporter.d., ...) created during the build process through the docs.

Adding this to the writing-documentation section would be appropriate.

To @dgozman , if this improvement direction is acceptable, please comment! I will submit the PR right away.

Thank you.

[pavelfeldman]

Thank you for your suggestion. This would explain one of a thousands things that are known to maintainers and not known to the new contributors, so I don't think additional details like this in contributors guide would systematically help.