ModularPipeline.save_pretrained: change the default `overwrite_modular_index` to be True (#14659)

* flipt default for save_pretrained overwrite_modular_index to be True

* update doc

* make style

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCdbvRpL9fv3h3WwSUPpfS

* Update docs/source/en/modular_diffusers/modular_pipeline.md

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Update docs/source/en/modular_diffusers/modular_pipeline.md

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Update docs/source/en/modular_diffusers/modular_pipeline.md

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Update docs/source/en/modular_diffusers/modular_pipeline.md

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* address review comments

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCdbvRpL9fv3h3WwSUPpfS

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>