🌱 Add .gitattributes to identify generated files to GitHub

[mdbooth]

Example: https://docs.github.com/en/repositories/working-with-files/managing-files/customizing-how-changed-files-appear-on-github

A primary is to stop labelling relatively small commits as XXL just because they update some doctext which is referenced in several places in the CRDs.

This does what I intend in git:

> git check-attr linguist-generated ./config/crd/bases/infrastructure.cluster.x-k8s.io_openstackclusters.yaml ./config/crd/kustomization.yaml
./config/crd/bases/infrastructure.cluster.x-k8s.io_openstackclusters.yaml: linguist-generated: true
./config/crd/kustomization.yaml: linguist-generated: unspecified

However, I'm far from convinced the github integration is working. I originally included it in https://github.com/kubernetes-sigs/cluster-api-provider-openstack/pull/1962, but it still seemed to be mislabelled.

/hold

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mdbooth

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/cluster-api-provider-openstack/blob/main/OWNERS)~~ [mdbooth] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[mdbooth]

CAPI are using the same attribute: https://github.com/kubernetes-sigs/cluster-api/blob/main/.gitattributes

[huxcrux]

/lgtm

[dulek]

Nice!

[mdbooth]

/hold cancel