provide _sidebar.ts for vitepress

[marco-porru]

Description

Changes proposed in this pull request:

• provide _sidebar.ts for vitepress

Pre-Merge Checklist

Consider all the following items. If your contribution violates any of them, or you are not sure about it, add a comment to the PR.

• The code coverage is acceptable.
• Release notes for the introduced changes are created.
• If Kubebuilder changes were made, you ran make generate-manifests and committed the changes before the merge.
• Pre-existing managed resources are correctly handled.
• The change works on all hyperscalers supported by SAP BTP, Kyma runtime.
• There is no upgrade downtime.
• For infrastructure changes, you checked if the changes affect the hyperscaler's costs.
• RBAC settings are as restrictive as possible.
• If any new libraries are added, you verified license compliance and maintainability, and made a comment in the PR with details. We only allow stable releases to be included in the project.
• You checked if this change should be cherry-picked to active release branches.
• The configuration does not introduce any additional latency.

Related issues
kyma-project/kyma#18820