docs(pages): Add responsive light and dark theme

[rahulbsw]

Summary

• make the documentation site light-first with a persistent light/dark theme toggle
• tighten desktop alignment and reduce unused whitespace while preserving responsive mobile navigation
• add the validated AWS sustained benchmark band to the homepage with a link to the full methodology

Scope

This PR contains only four documentation presentation files. It does not change StreamForge runtime code, deployment infrastructure, or benchmark results.

Validation

• Jekyll build and internal-link validation passed on this exact four-file redesign before branch creation
• desktop and mobile viewport checks passed without horizontal overflow
• theme selection persisted across reloads and produced no browser JavaScript errors
• git diff --check passed
• GitHub Pages PR build provides the clean-room build check