docs: add comparison handoff from architecture

[haosenwang1018]

Summary

• add a direct Comparison with Alternatives handoff after the homepage architecture overview
• help readers move from understanding how memsearch works into comparing that design with other approaches

Problem

Issue #91 is partly about discoverability. The homepage explains how memsearch works, but it does not give comparison-oriented readers an immediate next step into the alternatives page after they understand the model.

Changes

• add a short handoff line after the How It Works section in docs/index.md
• point readers to home/comparison.md

Fixes #91

Validation

• Python assertion check for the new link
• git diff --check