Inspector plugin documentation outdated

**Your Godot version:**
4.0 Alpha 13
**Issue description:**
`can_handle_object` and `parse_property` are now private.
`_parse_property` signature has changed.

**URL to the documentation page:**
https://docs.godotengine.org/en/latest/tutorials/plugins/editor/inspector_plugins.html?highlight=inspector%20plugin

[[If you know how to fix the issue you are reporting please
consider opening a pull request. We provide a tutorial on
using git here: https://docs.godotengine.org/en/stable/community/contributing/pr_workflow.html,
writing documentation at https://docs.godotengine.org/en/stable/community/contributing/docs_writing_guidelines.html
and contributing to the class reference here: https://docs.godotengine.org/en/stable/community/contributing/updating_the_class_reference.html
](https://docs.godotengine.org/en/latest/tutorials/plugins/editor/inspector_plugins.html?highlight=inspector%20plugin)](https://docs.godotengine.org/en/latest/tutorials/plugins/editor/inspector_plugins.html?highlight=inspector%20plugin)