MAINT: remove template module

We have quite a lot of very much functional and well-maintained modules atm, and I recommend following the logic (and API) in one of those when someone is proposing adding a new module. So my proposal would be to get rid of the `template_module` module. If there is anything left about the preferred API in there that is not included in the narrative docs, then move those parts to the docs.

Start by locating the `template_module` module and the narrative documentation, then compare its preferred API guidance with a maintained module. Done means the template module is removed and any guidance not already covered is incorporated into the narrative docs.

Written by the indexing model from the issue text.