Drop the experimental warning from Modular Diffusers (#14525)

Modular Diffusers is no longer experimental: remove the import-time warning
and the docstring warnings from ModularPipeline, the block base classes, and
every model's modular pipeline, and drop it from the docs overview.

ComponentsManager and the Mellon integration are still moving, so they keep
a warning — added one to the ComponentsManager docs page and to
mellon_node_utils, which had none.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>