Add documentation for hide_dpdjs and hide_dashboard

[andreialecu]

deployd/deployd#535

[ipepe]

where do You want me to add it?

[NicolasRitouet]

Maybe a new page: securing your deployd app?
@andreialecu what do you think?

[ipepe]

Well that is not really securing, a place that I would love to have is:

Customising Deployd configuration