Developer docs: reformat Virtual Environments description
Developer docs: reformat Virtual Environments description

Uses a ReST definition list for the top-level VM overview.
Expands a bit on the individual definitions to make attributes pop.

heartsucker/securedrop,ageis/securedrop,garrettr/securedrop,heartsucker/securedrop,ageis/securedrop,micahflee/securedrop,conorsch/securedrop,heartsucker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,micahflee/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,ageis/securedrop,conorsch/securedrop,ageis/securedrop,conorsch/securedrop,conorsch/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,micahflee/securedrop,micahflee/securedrop,garrettr/securedrop