Reimplement property object description for python

[tk0miya]

Is your feature request related to a problem? Please describe.
In Sphinx-2.1.0, we started to support property objects #6320. At that time, it is implemented as a kind of method. But we have some discussions #6785 and #7068 to change it to a kind of attribute.
Finally, we agreed to implement it as an independent type: property. At same time, it is very similar to attributes. So we also support to refer it as attributes.

Describe the solution you'd like

• Allow to describe property object via py:property directive.
• Allow to refer a property object via py:prop and py:attr role.
• The autodoc extension should render properties as py:property directive.
• The napoloen extension should render properties as py:property directive.
• The intersphinx extension should support to refer remote properties via py:prop and py:attr role.
• The autosumary extension generate a document that properties and attributes are shown in the same headings by default

[johann-petrak]

Not sure if this is the right place, but I think it would be extremely helpful if the generated documentation could not just show THAT something is a property, but also, indicate whether it has got a getter, setter, deleter.
This is important information for API users and manually documenting this is unnecessary (since it is easy to figure out automatically) and error prone (especially if e.g. a setter is added, since this would need to get documented in the getter, by convention).
One way to do this would be to show something like property/get/set or property/set/delete in the generate documentation.