Reorder Elasticsearch setup instructions

What does this Merge Request do?

Instruct the user to run rake gitlab:elastic:index before enabling Elasticsearch integration, because the integration cannot be enabled until the index is created.

Context

I ran into an issue with step 2 of the setup instructions, and following the steps in a slightly different order worked.

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• [-] This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.

/cc @mbergeron