docs: better installation instructions for the custom logo
docs: better installation instructions for the custom logo

It is confusing that the custom logo is mentionned twice in the
list of elements to prepare before running sdconfig, under two
slightly different names (custom header vs custom logo).

The details about the custom logo (size) are better included in a
dedicated section to keep the list short.

garrettr/securedrop,heartsucker/securedrop,heartsucker/securedrop,heartsucker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,garrettr/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,conorsch/securedrop,garrettr/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,garrettr/securedrop,heartsucker/securedrop