Add docstrings for autodoc

[fsbraun]

Description

Related resources

This PR adds documentation docstring for sphinx' autodoc feature to pull from the source code.

• In cms/models/pluginmodel.py/ it turns out that a defunct copy_plugin function was left over from versions 3.x
• This method is not covered by tests and in its original version must fail on each single call since it accesses treebeard properties of a plugin.
• This PR removes the CMSPlugin.copy_plugin function.
• For naming consistency, this PR renames the function create_title to create_page_content in cms/api.py. For ensure compatibility, a create_title function is retained. It issues a deprecation warning and calls create_page_content. This will require a change in the monkey patch routine of djangocms-versioning.

Checklist

• I have opened this pull request against develop
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[marksweb]

This looks good @fsbraun

It doesn't need it, but it could also test that the old create_title raises the deprecation warning.

[marksweb]

Looks good @fsbraun

The only thing I thought of was that there could be a test for ensuring create_title raises the warning.