feature: De-duplicate summary sections when sections are written manually and auto-summaries are activated

[pawamoy]

Is your feature request related to a problem? Please describe.

If I manually write an Attributes section, to give a specific order to attributes (or hide some of them), and I also activate the auto-summary option, I'll have two attributes sections rendered.

Describe the solution you'd like

Find a way to automatically de-duplicate sections: always keep the manually written ones. I suppose we can iterate on the object's docstring sections to check if there's already an attributes/function/etc. section.

Describe alternatives you've considered

Let the user handle it with the summary option. But this isn't fine-grained.

Additional context

Originally reported here: #133