Index GOV.UK support pages

We don't currently index the support pages on GOV.UK. So if you search for eg "Privacy Policy" it isn't returned. This PR changes that.

The API page is currently indexed as an external link, so should be removed:
https://github.com/alphagov/recommended-links/blob/master/data/index/recommended-link/internal_search_results.csv#L55

Copy changes are courtesy of @wryobservations