Update auth method documentation

[lgecse]

Follow up the discussion https://github.com/orgs/bank-vaults/discussions/2303

Please update the docs page regarding the auth methods, specially in terms of userpass auth.

[akijakya]

• supported auth methods that can be configured: https://github.com/bank-vaults/bank-vaults/blob/main/vault-config.yml
• auth methods supported by the webhook: https://github.com/bank-vaults/vault-sdk/blob/7090d894d587e7943a6f6a6809c260679602bac1/vault/client.go#L157-L186