In addition to jobs, there are other objects in Nomad that have a
specific format and can be provided to commands and API endpoints.

This commit creates a new menu section to hold the specification for
volumes and update the command pages to point to the new centralized
definition.

Redirecting the previous entries is not possible with `redirect.js`
because they are done server-side and URL fragments are not accessible
to detect a match. So we provide hidden anchors with a link to the new
page to guide users towards the new documentation.
Co-authored-by: Tim Gross <tgross@hashicorp.com>