DOC: fix relative path to userguide/index.md from docs/en/index.md

[MukundaKatta]

Summary

docs/en/index.md links to the User help guides using docs/en/userguide/index.md, which is a root-relative path rather than the relative path expected from the file's own location. When rendered on the Silverstripe docs site the link resolves to docs/en/docs/en/userguide/index.md and 404s.

Fixed by switching to the correct relative path userguide/index.md (the userguide/ folder is a sibling of index.md).

Closes #1425

Testing

Documentation-only change. Verified docs/en/userguide/index.md exists as a sibling directory of docs/en/index.md, so the new relative path resolves correctly both on GitHub and on the rendered docs site.

[MukundaKatta]

Withdrawing — low-value drive-by change. Apologies for the noise.