Add mlruns to gitignore to avoid pushing mlflow local runs to github

Description

I've noticed an increasing number of beginners facing some issues when using kedro and kedro-mlflow with the following workflow (see : https://github.com/Galileo-Galilei/kedro-mlflow/discussions/456):

They develop locally and start from a starer or the default template
They develop locally and mlflow creates a mlruns folder the first time they run a kedro pipeline
They push the local mlflow runs to a remote forge (Github / Gitlab)
If someone else pull the code on another machine, mlflow completely messes up artifacts path and they got troubles running their pipelines.

This all boils down to a misunderstanding of how mlflow and git works ; that said they are complex tools and beginners are often introduced to the whole tooling kedro / mlflow / git simultaneously, so it is understandable they do not get the point of not pushing data (their mlflow runs) to a remote forge. Besides, kedro-mlflow (and mlflow itself) do a lot of thing under the hood so some people do not even notice that a folder is created.

I suggest we ignore mlruns/* by default in kedro's template to avoid pushing data remotely, because that's the right thing to do (even if people use mlflow without kedro-mlflow) and that's hard to fix by kedro-mlflow (see https://github.com/Galileo-Galilei/kedro-mlflow/issues/523). I think a beginner will think twice before removing something from .gitignore if they don't really understand what is at stake.

Note: The change should also be done in starters.

Development notes

Add mlruns/* to gitgnore, to avoid pushing mlruns and all its subfolders in case it is created by kedro-mlflow. We may document why this folder should not be pushed, but it is likely to be done in kedro-mlflow, or mlflow iteslef.

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Checklist

[X] Read the contributing guidelines
[X] Signed off each commit with a Developer Certificate of Origin (DCO)
[ ] Opened this PR as a 'Draft Pull Request' if it is work-in-progress
[ ] Updated the documentation to reflect the code changes
[X] Added a description of this change in the RELEASE.md file
[ ] Added tests to cover my changes
[ ] Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team

kedro-org / kedro