docs: add modules to usagestats

[JTorreG]

Adds the modules object documentation to the Usage Statistics document

[lcrilly]

I know it's a different repo but want to make sure we don't forget to extend https://github.com/nginx/unit/blob/master/docs/unit-openapi.yaml#L3903

[lcrilly]

The overall context of the page (including the filename) is heavily predicated on usage statistics but the modules object is something a bit different.

E.g.

Unit collects instance- and app-wide metrics

Worth a look through the page to refine this explanation?

[ac000]

I know it's a different repo but want to make sure we don't forget to extend https://github.com/nginx/unit/blob/master/docs/unit-openapi.yaml#L3903

How is this used and is it hand generated?

[ac000]

The overall context of the page (including the filename) is heavily predicated on usage statistics but the modules object is something a bit different.

Should probably be renamed status-endpoint.rst or somesuch... (just status.rst might be confused for the overall status of the project)

[lcrilly]

I suggest statusapi.rst because we already have controlapi.rst

[lcrilly]

LGTM