feat: add root-level requirements.txt with all Python dependencies

[Gurram-sravani]

Summary

This PR adds a root-level requirements.txt file that
consolidates all Python dependencies needed to develop
and run the Kubeflow docs-agent locally.

Motivation

The README references many Python packages (pymilvus,
langchain, fastapi, sentence-transformers, kfp, etc.)
but there is no single requirements file at the root
level. New contributors must manually identify all
dependencies by reading through the code.

Changes

• Added requirements.txt with all core dependencies
• Grouped by category (Vector DB, LLM, API, Testing)
• Pinned minimum versions for stability

How to use

pip install -r requirements.txt

As a GSoC 2026 applicant for Project 1 (Agentic RAG
on Kubeflow), I noticed this gap while setting up my
local development environment.

[google-oss-prow]

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