docs: initial version of faq page (#911)

* docs: initial version of faq page

See #832

doc/md/faq.md

@@ -0,0 +1,44 @@
+---
+id: faq
+title: Frequently Asked Questions (FAQ)
+sidebar_label: FAQ
+---
+
+## Questions
+
+[How to create a struct (or a mutation) level validator?](#how-to-create-a-mutation-level-validator)
+
+[How to create an entity from a struct `T`?](#how-to-create-an-entity-from-a-struct-t)
+
+## Answers
+
+#### How to create a mutation level validator?
+
+In order to implement a mutation-level validator, you can either use [schema hooks](hooks.md#schema-hooks) for validating
+changes applied on one entity type, or use [transaction hooks](transactions.md#hooks) for validating mutations that being
+applied on multiple entity types (e.g. a GraphQL mutation).
+
+#### How to create an entity from a struct `T`?
+
+The different builders don't support the option of setting the entity fields (or edges) from a given struct `T`.
+The reason is that there's no way to distinguish between zero/real values when updating the database (for example, `&ent.T{Age: 0, Name: ""}`).
+Setting these values, may set incorrect values in the database or update unnecessary columns.
+
+However, the [external template](templates.md) option lets you extend the default code-generation assets by adding custom logic.
+For example, in order to generate a method for each of the create-builders, that accepts a struct as an input and configure the builder,
+use the following template:
+
+```gotemplate
+{{ range $n := $.Nodes }}
+    {{ $builder := $n.CreateName }}
+    {{ $receiver := receiver $builder }}
+
+    func ({{ $receiver }} *{{ $builder }}) Set{{ $n.Name }}(input *{{ $n.Name }}) *{{ $builder }} {
+        {{- range $f := $n.Fields }}
+            {{- $setter := print "Set" $f.StructField }}
+            {{ $receiver }}.{{ $setter }}(input.{{ $f.StructField }})
+        {{- end }}
+        return {{ $receiver }}
+    }
+{{ end }}
+```

doc/website/sidebars.json

@@ -31,7 +31,8 @@
       "templates",
       "graphql",
       "sql-integration",
-      "testing"
+      "testing",
+      "faq"
     ]
   }
 }