docs: docs: guide - default values cue

Add default values cue doc page + implement all maintainers suggestions

Signed-off-by: guillaume <guillaume.derouville@gmail.com>

docs/faq/1233-default-values-cue.md

@@ -0,0 +1,54 @@
+---
+slug: /1233/default-values-cue
+displayed_sidebar: '0.2'
+---
+
+# Default values and optional fields
+
+When writing a Cue config, you will sometimes want to set default values in your package.
+
+The most common way you'll encounter in our codebase is: `key: type | *value`:
+
+```cue
+defaultValue: string | *"foo"
+defaultValue: bool | *false
+```
+
+You'll also encounter the `*null` default value, which is self explanatory:
+
+```cue
+// here, defaultValue either accepts a #PersonalDefinition, or stays null by default
+defaultValue: #PersonalDefinition | *null
+```
+
+To test the type of `defaultValue`, you can directly do such assertion:
+
+```cue
+if defaultValue != "foo" | if defaultValue != false | if defaultValue != null {
+    ...
+}
+ 
+if defaultValue == "foo" | if defaultValue == false | if defaultValue == null {
+    ...
+}
+```
+
+However, don't get confused with the optional fields. Optional fields check whether a key is concrete at the given scope in the DAG. You declare them with `?` at the end of their name: `foo?`.
+
+```cue
+foo?: string // 1. declare foo. It remains undefined for now
+
+foo: "bar" // 2. Now, `foo` gets concrete. The field isn't undefined anymore
+```
+
+To check on a field's concreteness, use the bottom value `_|_`:
+
+```cue
+if foo != _|_ { // if foo is not `undefined`
+    ...
+}
+ 
+if foo == _|_ { // if foo is `undefined`
+    ...
+}
+```