docs: specify submodule types in README overviews

Updated all 54 subdirectory README files to use specific module type names instead of generic 'This module':

- API modules now say 'This API module'
- Utils modules now say 'This utils module'
- Forms modules now say 'This forms module'
- Models modules now say 'This models module'
- Classes modules now say 'This classes module'
- Constants modules now say 'This constants module'
- Management modules now say 'This management module'
- Commands modules now say 'This commands module'
- Third-party integrations now say 'This integration'

This makes the documentation clearer about what each submodule is, especially for nested modules like apps/accounts/api/ which belongs to the accounts app.

Author: YosefAshenafi