Update web dashboard documentation for new default behavior

**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](https://www.google.com/search?q=https://github.com/grafana/k6/pull/4913)

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