Link main README installation to storefront docs

[SiddharthGautam040]

Fixes #6516

Summary

The root README duplicated an outdated Solidus installation flow (solidus:install without --frontend=starter) and a sprockets manifest workaround. The storefront README is the canonical install path.

This PR removes the duplicated steps from the root README and links to storefront/README.md instead. Post-install info (Imagemagick prerequisite, server URLs, default admin credentials, and support links) stays in the root README.

Test plan

• Verified the link to storefront/README.md renders correctly
• Confirmed storefront README contains the current install flow with --frontend=starter

Checklist

The following are mandatory for all PRs:

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have localized any and all user-facing strings that I added to the source code. (N/A — no user-facing strings added)
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD. (N/A — docs-only change)
• 🛣️ I have opened a PR to update the guides. (N/A — no guide changes needed)
• ✅ I have added automated tests to cover my changes. (N/A — documentation-only change)
• 📸 I have attached screenshots to demo visual changes. (N/A — no visual changes)