Improve docs: add workflow example and common issues section

[TALBIYA-PARVEEN]

Description

This PR improves the documentation for modifying and querying data by:

• Adding a complete end-to-end workflow example demonstrating create, read, update, and delete operations in a single flow.
• Introducing a "Common Issues" section that highlights frequent problems such as working outside the application context, forgetting to commit changes, and incorrect database configuration.

These additions aim to improve clarity and developer experience, especially for beginners using Flask-SQLAlchemy.

Notes

• This is a documentation-only change, so no tests are required.
• Changes follow existing documentation style and formatting.

It looks like the Read the Docs build is failing due to a missing Sphinx configuration key. This appears unrelated to the changes in this PR, as the failure occurs during the configuration stage.

Please let me know if any adjustments are needed from my side.