Added an example credential with HashiCorp Vault source

[ryangniadek]

What does this PR do?

Currently, the documentation for the credential_input_sources role does not provide an example for HashiCorp Vault, which requires different metadata fields from the provided CyberArk example.

• This PR adds an example of using HashiCorp Vault as a credential input source.
• This PR also adds an example of creating the HashiCorp Vault source credential using the credentials role.

How should this be tested?

This should be tested by comparing the provided example with working config as code implementations that use HashiCorp Vault as a credential input source

Is there a relevant Issue open for this?

No

Other Relevant info, PRs, etc

• We used roles within this collection with HashiCorp Vault credentials and had a hard time finding documentation. Ideally, all supported credential plugins would be documented and this gets us one step closer.
• It is debatable whether adding the source credential is in scope for this PR. Those changes are contained in b69ee64 if it's decided to remove them from the PR.

[ivarmu]

LGTM