docs: add open-bao readme compatibilies overwiew

[SoulKyu]

@Lujeni

[SoulKyu]

API that has been tested :

  Authentication & Health:
  - client.Auth().Login() - Kubernetes auth login (pkg/vault/vault.go:102)
  - client.Sys().HealthWithContext() - Health check (pkg/vault/vault.go:120)

  Token Management:
  - client.Auth().Token().CreateOrphanWithContext() - Create orphan tokens (pkg/vault/vault.go:138)
  - client.Auth().Token().RenewWithContext() - Renew tokens (pkg/vault/handle_token.go:406)
  - client.Auth().Token().RenewSelfWithContext() - Renew own token (pkg/vault/handle_token.go:431)
  - client.Auth().Token().RevokeOrphanWithContext() - Revoke tokens (pkg/vault/handle_token.go:377)
  - client.Auth().Token().RevokeSelfWithContext() - Revoke own token (pkg/vault/handle_token.go:395)

  KV Storage (v2):
  - client.KVv2().Put() - Store credential metadata (pkg/vault/handle_token.go:60)
  - client.KVv2().Delete() - Delete secrets (pkg/vault/handle_token.go:126)
  - client.KVv2().DeleteMetadata() - Delete metadata (pkg/vault/handle_token.go:132)
  - client.Logical().ReadWithContext() - Read secrets (pkg/vault/handle_token.go:152, 216)
  - client.Logical().ListWithContext() - List secrets (pkg/vault/handle_token.go:184)

  Database Engine:
  - client.Logical().Read() - Get database credentials (pkg/vault/vault.go:214)

  Lease Management:
  - client.Sys().LookupWithContext() - Check lease info (pkg/vault/handle_token.go:444)
  - client.Sys().RenewWithContext() - Renew leases (pkg/vault/handle_token.go:461)

[Lujeni]

closes #49 🚢