feat(httpd) allow using subdir for the htdocs volume mount

[dduportal]

Related to https://github.com/jenkins-infra/helpdesk/issues/2649#issuecomment-2364054995

This PR allows the chart to mount the htdocs directory from a sub directory in the persistent volume.

• Unit tests added
• Tested with success with an emptyDir on a local k3d ephemeral cluster
• Tested with success by installing a new HTTPD chart using a subdir on production (with the Azure File storage created in https://github.com/jenkins-infra/azure/pull/840