[modular] Simplify logic and docstring handling

[Cyrilvallez]

What does this PR do?

Simplify a lot replace_class_node logic (I wanted to do it from a long time ago as it was all bloated and super hard to read) and simplify the docstring handling at the same time.

Basically, the idea is now to use the parent docstring if there are no docstrings, or to use the docstring in modular if any. Previously, we were doing some kind of merging between the 2 docstrings, which was bugged (see a few places in the diff where there are arg explanations after examples or similar issues), inconsistent (i.e. adding a simple arg for a Config, while convenient, was always inconsistent because the checkpoints link was never updated correctly) and surprising (this is some kind of magic that contributors were very rarely understanding (which makes sense at it had a lot of issues as explained). Moreover, with auto_docstring now, we should not have to redefine the parameters explanations everytime, so it makes sense to relax docstring handling.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: d_fine, diffllama, dinov2_with_registers, emu3, falcon_h1, gemma, gemma2, gemma3, gemma3n, glm4v, helium, instructblipvideo, internvl, mlcd, sam_hq, smolvlm