show how to use the JWT for OIDC auth in the docs

[lawndoc]

It wasn't immediately clear how to use the OIDC authentication method from the provider documentation. Currently, you need to read the source code to uncover the environment variable that is needed.

[lawndoc]

References:

terraform-provider-infisical/internal/client/login.go

Lines 50 to 51 in 342fb1e

	func (client Client) OidcMachineIdentityAuth() (string, error) {
	authJwt := os.Getenv(INFISICAL_AUTH_JWT_NAME)

terraform-provider-infisical/internal/client/constants.go

Line 12 in 342fb1e

INFISICAL_AUTH_JWT_NAME = "INFISICAL_AUTH_JWT"