Docs: update UIAlert import path and usage for v3 (#58089)

[Lohith625]

---

This PR updates the documentation in howto/customize-ui.rst to reflect the correct import path and usage of the UIAlert class for Airflow v3.

Previously, the documentation showed:

from airflow.www.utils import UIAlert

DASHBOARD_UIALERTS = [
    UIAlert("Welcome to Airflow"),
]

In Airflow v3, UIAlert has moved to the airflow.api_fastapi.common.types module and requires keyword arguments text and category.

The updated example is:

from airflow.api_fastapi.common.types import UIAlert

DASHBOARD_UIALERTS = [
    UIAlert(text="Welcome to Airflow", category="info"),
]

[Lohith625]

Hi @lp-jump 👋
I’ve updated the UIAlert example in howto/customize-ui.rst as suggested.
While reviewing the docs, I also noticed a few other occurrences of airflow.www.utils in different sections.
Would you like me to update those as well to align with the v3 structure, or should we keep this PR limited to just the UIAlert example?

Happy to make the additional changes if needed. 😊