Add workflow to prevent changes to Extensions list

[heitortsergent]

What?

Add a workflow to prevent users from changing the explore.md file. The file should only be updated by maintainers, or the Extension Registry workflow in this repo.

Checklist

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the npm start command locally and verified that the changes look good.

• I have made my changes in the docs/sources/k6/next folder of the documentation.
• I have reflected my changes in the docs/sources/k6/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the relevant folders of the two previous k6 versions of the documentation (if still applicable to previous versions).

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

Related PR(s)/Issue(s)

#1961

[heitortsergent]

@codebien would you be able to help me with this? 🙏

I'm wondering if this looks right / if the workflow should be running on this PR already. 🤔