[docs] Organization limits screenshot is outdated

[nemesifier]

This image:

https://github.com/openwisp/openwisp-controller/blob/docs/docs/1.1/organization-limits.png

Is missing the configuration variables field (context, was probably added later in the same release).
We shall update it.

[omachala]

Hi! I can help update this screenshot.

I maintain heroshot, a screenshot automation tool that captures screenshots from a config file using Playwright. Once set up, regenerating screenshots becomes a one-command operation.

Happy to submit a PR!