docs(neosantara): add provider documentation

[ErRickow]

Summary

• add Neosantara provider documentation
• document chat completions, streaming, responses, function calling, proxy usage, and parameter compatibility
• add the provider page to the docs sidebar

Verification Notes

• live-tested against Neosantara using LiteLLM pre-release 1.89.0rc1
• verified neosantara/gemini-3.5-flash for completion(), streaming, responses(), function calling, max_completion_tokens, and reasoning_effort
• docs repo build was not run locally in this environment because node_modules is not installed in this checkout

Context

• runtime support was verified from the pre-release package rather than the latest stable PyPI release at the time of testing

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Project	Deployment	Actions	Updated (UTC)
litellm	Ready	Preview, Comment	Jun 7, 2026 6:34am

[ErRickow]

@mateo-berri hi, since litellm already latest with version 1.89.0 and have Neosantara provider on it, would you like to proceed this docs for me? :)

[ErRickow]

related PR: BerriAI/litellm#29646