add mention to helpers page in workshop website template

The workshop website template has now a dedicated page (at /helpers/index.html, e.g., ) that has some useful information for helpers. It might be useful to have it mentioned during instructor training.

Review the instructor-training materials and the workshop website template's helpers page at https://carpentries.github.io/workshop-template/helpers/index.html. Find where helper resources are introduced, add a mention or link to that page, and verify the rendered training material points to the intended page.

Written by the indexing model from the issue text.