Indicate usage of the re2 regex engine in the .airflowignore documentation.

[tshirtman]

As the engine was changed in #32303, and the feature set of this engine is a bit specific, it seems useful to be explicit about it.

Added a short explanation about it in the dags documenatiton, with a link to the re2 syntax wiki page.

related: #32303

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our pre-commits will help you with that.
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[potiuk]

You need to use double backticks in .rst :)

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.