[doc] Add requirement for gettext and graphviz
When running the tutorial from scratch, I had errors because these
two packages were not installed.
option_group_changed('port', 'main', 'web')
option_group_changed('query-log-file', 'main', 'web')
option_group_changed('profile', 'main', 'web')
option_group_changed('max-post-length', 'main', 'web')