Basic graduation of k6/timers docs

[mstoykov]

What?

Add k6/timers that are just copy of k6/experimental/timers

More work is needed for this docs to be usable, but #1036 seems really relevant and is taking too long and might not fit in the v0.50.0 release cycle - so this the bare minimum so we can have the docs out.

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

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