📌 ISSUE-#199: Remove external dotflow-mongodb references

[FernandoCelmer]

Description

Remove all references to the external dotflow-mongodb package, preparing the codebase for a native MongoDB provider implementation.

Issue: 📌 ISSUE-#199

Changes

• Remove dotflow/storage.py legacy module (replaced by dotflow.providers)
• Remove _MongoDBModuleNotFound class and dotflow-mongodb external import
• Remove docs_src/storage/storage_mongodb.py example
• Update examples submodule: remove step_with_storage_mongodb.py, dotflow-mongodb dependency, and update imports to dotflow.providers

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• My changes generate no new warnings