Summary

• Update the getting-started HTML example to reference index.scss instead of placeholder path/to/index.scss.
• Clarify that the referenced index.scss file may be empty but must exist next to index.html.

Fixes #958

Test plan

• git diff --check
• Searched the site content to verify the stale path/to/index.scss example was removed.
• zola check was not run locally because zola is not installed in this environment.