Update web dashboard documentation for new default behavior

[szkiba]

Why

The behavior of the built-in k6 web dashboard will be changed in the grafana/k6 repository. To improve the out-of-the-box user experience, it will be enabled by default once the related PR is merged.

This change will introduce a new opt-out mechanism and deprecate the old opt-in controls, requiring documentation updates to prevent user confusion upon release.

What needs to be updated?

The documentation will need to be updated to reflect this new behavior. The key changes are:

1. The dashboard will be enabled by default. The documentation should no longer state that it needs to be explicitly enabled with an environment variable.
2. The opt-in controls will be removed. All references to K6_WEB_DASHBOARD=true and the webDashboard: true config option should be removed or updated.
3. New opt-out controls will be introduced. The documentation must clearly explain how to disable the dashboard using:
   • Environment variable: K6_NO_WEB_DASHBOARD=true
   • JSON config property: noWebDashboard: true

Additional context

This documentation update is for the feature change being implemented in grafana/k6. The related pull request can be found here for more technical context: grafana/k6#4913

Aligning the documentation with this new behavior before it is released is critical for a smooth user transition.