Update docs: add core.md and newtype.md - refined4s-core and Newtype

docs/core/_category_.json

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+{
+  "label": "refined4-core",
+  "position": 2,
+  "collapsible": true,
+  "collapsed": false
+}

docs/core/core.md

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+---
+id: core
+title: "core module"
+---
+
+import DocCardList from '@theme/DocCardList';
+
+## Import
+```scala mdoc
+import refined4s.*
+import refined4s.syntax.*
+```
+
+<DocCardList />

docs/core/newtype/_category_.json

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+{
+  "label": "Newtype",
+  "position": 1,
+  "collapsible": true,
+  "collapsed": false
+}

docs/core/newtype/newtype.md

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+---
+sidebar_position: 1
+id: newtype
+title: "Newtype"
+---
+
+## `Import`
+```scala mdoc
+import refined4s.*
+```
+
+## Define Newtype
+
+```scala
+type NewtypeName = NewtypeName.Type
+object NewtypeName extends Newtype[ActualType]
+```
+
+e.g.)
+```scala
+type Name = Name.Type
+object Name extends Newtype[String]
+```
+
+## Create Value
+
+```scala
+val newtypeName = NewtypeName(value)
+```
+
+## Get Actual Value
+
+```scala
+newtypeName.value
+```
+
+
+## Example
+```scala mdoc
+type Name = Name.Type
+object Name extends Newtype[String]
+
+type Email = Email.Type
+object Email extends Newtype[String]
+
+def hello(name: Name): Unit = println(s"Hello ${name.value}")
+
+def send(email: Email): Unit = println(s"Sending email to ${email.value}")
+
+val name = Name("Kevin")
+// Name.Type = "Kevin"
+name.value
+
+hello(name)
+
+val email = Email("[email protected]")
+// Email.Type = "[email protected]"
+email.value
+
+send(email)
+```
+```scala mdoc:fail
+hello("Kevin")
+```
+```scala mdoc:fail
+send("[email protected]")
+```