feat(key-manager): add a documentation for key rotation

[mmrqs]

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Description

Add a documentation in order to provide an example for the key rotation

[RoRoJ]

Hi :-) How Tos are concerned with how to do things via the Scaleway console - I think this doc should be in the API/CLI section. https://www.scaleway.com/en/docs/guidelines/#page-types

[nerda-codes]

Hello, I'm closing all the Key Manager PRs to integrate them with all the other documentation pages. This one will be available in the regular documentation website, under the Key Manager section.