docs: Integration with Nebius AI Studio (#31293)

Thank you for contributing to LangChain!

[x] PR title: langchain_ollama: support custom headers for Ollama
partner APIs

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Use "docs: ..." for purely docs changes, "infra: ..." for CI changes.
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[x] PR message:

**Description: This PR adds support for passing custom HTTP headers to
Ollama models when used as a LangChain integration. This is especially
useful for enterprise users or partners who need to send authentication
tokens, API keys, or custom tracking headers when querying secured
Ollama servers.
Issue: N/A (new enhancement)
**Dependencies: No external dependencies introduced.
Twitter handle: @arunkumar_offl
[x] Add tests and docs: If you're adding a new integration, please
include
1.Added a unit test in test_chat_models.py to validate headers are
passed correctly.
2. Added an example notebook:
docs/docs/integrations/llms/ollama_custom_headers.ipynb showing how to
use custom headers.

[x] Lint and test: Ran make format, make lint, and make test to ensure
the code is clean and passing all checks.

Additional guidelines:

Make sure optional dependencies are imported within a function.
Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
Most PRs should not touch more than one package.
Changes should be backwards compatible.
If no one reviews your PR within a few days, please @-mention one of
baskaryan, eyurtsev, ccurme, vbarda, hwchase17.

This MR is only for the docs. Added integration with Nebius AI Studio to
docs. The integration package is available at
[https://github.com/nebius/langchain-nebius](https://github.com/nebius/langchain-nebius).

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Co-authored-by: Akim Tsvigun <[email protected]>
Co-authored-by: Chester Curme <[email protected]>

Author: 3 people