Improve invalid response error messages

[kavasg]

Description

This PR enhances error messages for invalid return types from view functions to make them more helpful for developers, especially those new to Flask.

When a view function returns an unsupported type, the current error message provides basic information but lacks comprehensive guidance about what return types are actually supported and how to format them correctly.

Changes

• Expanded the error message in make_response() to include:
  • Clear descriptions of all valid return types
  • Examples of the correct format for each type
  • A link to the relevant documentation
  • Better formatting with newlines for readability

Why This Matters

These improved error messages will significantly enhance the developer experience by:

• Reducing debugging time when encountering this common error
• Providing immediate guidance without requiring a search through documentation
• Helping new Flask users learn the framework's patterns more quickly

Changelog Entry

I'm not sure if this change warrants a changelog entry as it's a developer-facing enhancement rather than a user-facing feature or fix. If maintainers feel it should be included, I can add the following line:

• Improved error messages for invalid return types from view functions, providing clearer guidance about supported return formats.

Related Issues

While working with Flask in my own project, I encountered this error message and found myself having to look up the documentation to understand what return types were valid. I believe improving this error message will help other developers avoid the same confusion, especially those who are new to Flask.
I'm submitting this PR without an associated issue since it's a simple improvement to error messaging/documentation rather than a functional change, as per the contribution guidelines.

[kavasg]

Update: Fix ReadTheDocs Build 🛠️

In addition to the original changes, this PR now includes a fix for the ReadTheDocs build failure.

🔍 Issue

ReadTheDocs recently deprecated automatic configuration detection, requiring an explicit sphinx.configuration key in .readthedocs.yaml. This was causing all PR builds to fail.

✅ Fix (Second Commit)

• Added sphinx.configuration: docs/conf.py to .readthedocs.yaml.
• Ensures that ReadTheDocs correctly finds the Sphinx configuration.
• This should resolve documentation build failures across all PRs.

🔄 Impact

With this change, ReadTheDocs builds should now pass as expected, allowing this PR to proceed smoothly.