Keep experiments doc up-to-date

[clearfeed]

It'd be nice if we can keep this doc up-to-date with the list of experiments we are currently running, and what they do https://coder.com/docs/contributing/feature-stages.

Looking at the doc, I thought there's currently no experiments

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Issue created in Slack by matifali from this thread in #shared-coder-canva.

[matifali]

@stirby for further action.

[stirby]

We already do this.
#13872