Doucment how to configure Vault Client logging level

Problem/Motivation

IN 📌 Add filter for logs from \Vault\CachedClient Active we added the vault.logger.level container parameter to filter the logs emitted to the system log.

We did not however document the parameter. We should add documentation for this to aid developers and site owners.

Steps to reproduce

N/A

Proposed resolution

Add documentation to GitLab Pages.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None