docs/config: add documentation for the readonly-host setting
docs/config: add documentation for the readonly-host setting

Also improve the structure of the document.

tsuru/gandalf,tsuru/gandalf,gleez/gandalf,scorphus/gandalf,flaviamissi/gandalf,gleez/gandalf,flaviamissi/gandalf,marcelometal/gandalf,gleez/gandalf,marcelometal/gandalf,scorphus/gandalf,marcelometal/gandalf