Migrate content from API to UG - modules 1, 2, 3, 4 #417

Description:

As part of the MDAnalysis SDG23 project, we are performing a cleanup and integration of content within the API documentation. The goal is to differentiate between the API Guide (auto-generated references) and the User Guide (narrative, how-to, and conceptual documentation).

Steps:

Review API documentation.
Create a document that categorizes content into
Content to retain in the API docs
Content to migrate to the User Guide
Begin content migration and cleanup accordingly.