DOC: drop the experimental tag constrained_layout and tight_layout

[jklymak]

PR Summary

constrained_layout was marked experimental. Its been around for a couple of years with no indication that the API will change, and no complete show-stoppers so I propose to drop the experimental tag.

Interestingly, as I grepped for "experimental" tight_layout was deemed experimental as well.

Both tutorials had useful information in the same clause as the experimental designation, so I tried to maintain that info in the rewrite.

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[jklymak]

I guess this should get two approvals because it is dropping an "experimental" tag.

[timhoffm]

IMHO we should mention the transition to a stable feature in a what's new or API change entry.

[jklymak]

Sure, what's-new entry added.

[QuLogic]

If @jklymak thinks it's not experimental, that's good enough for me.