[docs] [issue36971] Add subsections in C API "Common Object Structures" page

[Jeroen Demeyer]

Change by Jeroen Demeyer <J.Demeyer at UGent.be>:


----------
keywords: +patch
pull_requests: +13354
stage:  -> patch review

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36971>
_______________________________________