docs: start documenting previewers (#574)

Co-authored-by: Muhammed Zakir <MuhammedZakir@users.noreply.github.com>

scripts/gendocs.lua

@@ -12,6 +12,7 @@ docs.test = function()
     "./lua/telescope/builtin/init.lua",
     "./lua/telescope/pickers/layout_strategies.lua",
     "./lua/telescope/actions/init.lua",
+    "./lua/telescope/previewers/init.lua",
   }
 
   table.sort(input_files, function(a, b)