DOC: Remove Dask and Modin sections in `scale.rst` in favor of linking to ecosystem docs.

[mroeschke]

xref #57586 (review)

1. Remove Use Dask and Use Modin sections in doc/source/user_guide/scale.rst
2. Add a new section in doc/source/user_guide/scale.rst (Use Other Libraries) and link to Out-of-core section in web/pandas/community/ecosystem.md
3. Remove dask-expr in environment.yml and requirements.txt
4. Remove version pinning on dask post DOC: Pin dask/dask-expr for scale.rst #57830

[yukikitayama]

take

[yukikitayama]

Hi @mroeschke I don't know which code I need to update regarding "4. Remove version pinnig on dask post". Sorry if I'm asking too basic. I'm new to contribution.

[mroeschke]

environment.yml and requirements.txt