Add methods deprecated in v0.120.0

Closes #2323

content/en/methods/site/IsDevelopment.md

@@ -0,0 +1,20 @@
+---
+title: IsDevelopment
+description: Reports whether the current running environment is “development”.
+categories: []
+keywords: []
+action:
+  related: []
+  returnType: bool
+  signatures: [SITE.IsDevelopment]
+# deprecated 2023-10-30
+expiryDate: 2024-10-30 
+_build:
+  list: never
+---
+
+{{% deprecated-in 0.120.0 %}}
+Use [`hugo.IsDevelopment`] instead.
+
+[`hugo.IsDevelopment`]: /functions/hugo/isdevelopment
+{{% /deprecated-in %}}