Submodules Should Allow for Overriding Plugin Options

I have a use case where my monorepo contains a number of different projects, each being rendered using `mkdocstrings`. The owners of various projects want different configuration settings for `mkdocstrings` however this plugin only combines the `nav` section. I wanted to know if there was a way to allow it to merge the `plugins` as well? If not this could be a helpful feature.

Start by reviewing the plugin's current submodule configuration handling and how the nav section is combined. Determine the intended precedence for project-level plugin options, then add coverage for the merge behavior and verify that different documentation projects can retain distinct mkdocstrings settings.

Written by the indexing model from the issue text.