Document kubernetes auth method for Vault

[stgrace]

Description of feature

Document vault's kubernetes auth method. This auth method is required when using stackl's vault secret handler. Currently this is not documented.

Write documentation on how to enable the kubernetes auth method and what env variables need to added to the stackl-agent configuration in order for it to work

Purpose of this new feature

Make the use of vault in stackl clear to users and create documentation for users to use vault as a secret handler in stackl