Create a stale script to mark stale issues and pull requests

[varodrig]

Since we currently have 79 issues open (the oldest is from 2020).
https://github.com/kubeflow/website/pulls

We need a way to standarize PRs and issues to be stale after x amount of time. This was mentioned during the community meeting last week.

The recommended implementation is to follow a similar implementation as the pipeline module does:

• https://github.com/kubeflow/pipelines/blob/master/.github/workflows/stale.yml

[varodrig]

Please let me know if you are ok with this, so I can proceed with the implementation @andreyvelich @mameshini @hbelmiro

[hbelmiro]

Sounds good to me.

[andreyvelich]

Sounds good, thanks @varodrig.
Please also add message to stale PRs, similar to our Training Operator stale config: https://github.com/kubeflow/training-operator/blob/master/.github/workflows/stale.yaml#L34-L43