ENH: public API render methods to PlotContainer and FixedResolutionBuffer

[neutrinoceros]

PR Summary

A simple addition to the public interface would be useful in #4118 to avoid using private methods directly.
Content:

• ENH: add a PlotContainer.render method (public interface for _setup_plots)
• DOC: avoid using private _setup_plots method in user docs

For backward compatibility, I avoided simply renaming _setup_plots.
Hopefully the intented usage is clear from the docstring and the docs update.

I'm not completely certain that my use of the @validate_plot is warranted or justified, and I would particularly appreciate feedback specifically on this.

[neutrinoceros]

just noticed that FixedResolutionBuffer could also use a render method to clarify usage of fib[item], I'll include a patch for this too

[matthewturk]

I like the idea, but I'm not sure why we'd want to add @validate_plot here if it's not required by _setup_plots.

[neutrinoceros]

yup I actually don't think it's needed.

[neutrinoceros]

@matthewturk I removed the unnecessary decorator, this is now ready for review again