[RFC] Reorganize project structure to prioritize Kagent and MCP

# [Refactor] Restructure Repository to Prioritize Kagent and Deprecate Legacy Servers

## Description
As the project evolves towards a more modern, agentic RAG architecture (as outlined in `gsoc2026_agentic_rag.md`), the current repository structure contains several monolithic server implementations that are no longer the primary focus. This issue tracks the necessary repository restructuring to reflect our new architectural direction.

## Proposed Changes
- [ ] **Deprecate Legacy Servers:** Move the existing `server/` and `server-https/` implementations into a new `legacy-servers/` directory. This will clearly signal their outdated status to developers while safely maintaining backward compatibility.
- [ ] **Prioritize Kagent in Documentation:** Update the root `README.md` to explicitly highlight the `kagent-feast-mcp/` setup as the recommended, primary deployment path.

## Reasoning
This restructuring strongly aligns with the recent community direction toward `kagent` and the Model Context Protocol (MCP). Implementing these changes will provide several benefits:
* **Cleaner Workspace:** Ensures a much cleaner and more navigable root directory.
* **Improved Onboarding:** Provides a streamlined onboarding experience for new users and contributors by pointing them directly to the current architectural targets.
* **Reduced Confusion:** Eliminates ambiguity between the older monolithic server approaches and the new agentic RAG setup.

## References
* Ref: [Community feedback regarding the kagent transition (PR #145)](https://github.com/kubeflow/docs-agent/pull/145#issuecomment-4090203798)

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*Note: This architectural update is proposed as part of my Google Summer of Code (GSoC) 2026 project.*

Start by reading gsoc2026_agentic_rag.md and inspecting server/, server-https/, kagent-feast-mcp/, and README.md. Check repository references affected by moving the legacy servers, then confirm that both implementations remain available under legacy-servers/ and that README.md presents kagent-feast-mcp/ as the recommended deployment path.

Written by the indexing model from the issue text.