add: project explore api

app/models/project_category.rb

@@ -8,6 +8,12 @@
 #  projects_count :integer          default("0")
 #  created_at     :datetime         not null
 #  updated_at     :datetime         not null
+#  ancestry       :string(255)
+#  pinned_index     :integer          default("0")
+#
+# Indexes
+#
+#  index_project_categories_on_ancestry  (ancestry)
 #
 
 class ProjectCategory < ApplicationRecord