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The root README duplicated an outdated Solidus installation flow (solidus:install without --frontend=starter) and a sprockets manifest workaround.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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.02%. Comparing base (d4463ad) to head (bcba8e5).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6517   +/-   ##
=======================================
  Coverage   92.02%   92.02%           
=======================================
  Files        1035     1035           
  Lines       21130    21130           
=======================================
  Hits        19445    19445           
  Misses       1685     1685           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.