updates KMS support docs

[ChrisJBurns]

• updates and migrates KMS support docs as part of documentation migration Migrate documentation to docs.sigstore.dev cosign#822
• corrects reference to dlorenc images and replaces with generic user
• improves readability for some code snippets
• adds mention for Azure Key Vault and Kubernetes Secrets

This aims to deprecate https://github.com/sigstore/cosign/blob/main/KMS.md

Signed-off-by: ChrisJBurns 29541485+ChrisJBurns@users.noreply.github.com

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[ChrisJBurns]

@ltagliaferri Here is the KMS one for whenever you have some time :) I've pretty much just copied and pasted the one from sigstore/KMS.md and just improved some formatting to make the page a bit more readable.

[ltagliaferri]

Awesome, thanks!