Document Vault set up process for bound-claims, policies, and roles

Problem to solve

We need to reduce the set up confusion for Vault by improving the documentation on how to configure bound-claims, policies, and roles

Proposal

1. Add a cross-link documentation to what bound claims are in HashiCorp documentation - https://www.vaultproject.io/docs/auth/jwt
2. Expand example documentation with more instructions on HashiCorp policies in External Secrets - https://docs.gitlab.com/ee/ci/secrets/

For example, adding content around https://www.vaultproject.io/docs/concepts/policies