Implement and document alpha/beta_feature_available? method

What does this MR do?

This documents the Namespace#beta_feature_available? method, and implements an equivalent alpha_feature_available? method.

An earlier, related MR !19700 (merged) added information about preceding feature_available? calls with Feature.enabled? to avoid making half-finished features available on self-managed instances (since we can't explicitly disable the flag there, but we can on GitLab.com).

I think alpha/beta_feature_available? is a better choice, since it's only one call, and its name better communicates that the feature isn't ready yet.

Related MRs

!19700 (merged)

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