(Maint) Fix link to the REST API docs

When the API docs were moved to a new directory structure the link to
them wasn't updated.
(#22591) Document /env/file_content/*

This documents how the make requests and understand responses from the
file_content endpoint.
Merge branch 'issue/master/22591-document-file_content'

* issue/master/22591-document-file_content:
  (#22591) Document /env/file_content/*
  (Maint) Fix link to the REST API docs