Rename "TeSS Platforms" as "TeSS instances" #4

In the "In Use" section, there is a content type "TeSS Platforms" tess-platform with several pages.

This section should be called "TeSS Instances" tess-instances.

Rename this content type and update all the pages affects.
Update the tess-platforms-index to tess-instances-index.
Update the sidebar menu.