docs: update documentation links

[Lovlace777]

Summary

• Updates the README documentation links from the old luciofranco.github.io host to the current jennings.io/jj-spr site.
• Fixes the main documentation entry and all quick links in the Documentation section.
• Keeps the change scoped to README.md.

Validation

• curl.exe -L -sS -o NUL -w "%{http_code} %{url_effective}" for the documentation index and all seven quick links: all returned 200.
• git diff --check -- README.md: passed.
• Select-String -Path README.md -Pattern 'luciofranco.github.io/jj-spr' -SimpleMatch: no remaining matches.

Notes

• Fixes Links to documentation broken #137.
• README-only change; no runtime compatibility impact.
• I did not run cargo test, cargo clippy, cargo fmt --check, or mdbook build locally because cargo and mdbook are not installed in this environment, and the change only updates external documentation links.