New page for Key Vault

Use content from here: https://github.com/cse-labs/opsplaybook/blob/main/docs/code-with-devsecops/Contributions/keyvault-access-with-rbac.md to create an all-up working w/ Key Vault page. Should be linked off of the main secrets mgmt page.

Put it in folder according to: https://github.com/microsoft/code-with-engineering-playbook/issues/1004