Web dashboard documentation updated to reflect the default enabled state

[szkiba]

What?

This PR updates all relevant documentation to reflect that the built-in web dashboard is now enabled by default. The core concept has been shifted from "opting-in" to "opting-out."

Key changes include:

• The main /results-output/web-dashboard page has been rewritten to explain the new default behavior.
• Instructions and examples have been updated to use the new opt-out mechanism (K6_NO_WEB_DASHBOARD).
• All references to the old opt-in methods have been removed.

Why

This documentation update is necessary to align with a core feature change in an upcoming release of k6. The web dashboard will now run by default, and failing to update the documentation would cause significant confusion for both new and existing users. This PR ensures that the documentation is accurate and reflects the new, improved user experience at the time of release.

Checklist

• [x] I have used a meaningful title for the PR.

• [x] I have described the changes I've made in the "What?" section above.

• [x] I have performed a self-review of my changes.

• [x] I have run the npm start command locally and verified that the changes look good.

• [x] I have made my changes in the docs/sources/k6/next folder of the documentation.

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Related PR(s)/Issue(s)

Related to: grafana/k6#4913

Closes: #2001

[CLAassistant]

All committers have signed the CLA.