add iOS build instructions

docs.json

@@ -17,6 +17,7 @@
       ["Showcase", "/showcase"],
       ["Windows", "/windows"],
       ["Android", "/android"],
+      ["iOS", "/ios"],
       ["Web", "/web"],
       ["Contributing", "/contributing"],
       ["Discord", "https://discord.gg/h2VdDK3EAQ"]

docs/ios.mdx

@@ -0,0 +1,12 @@
+## iOS
+
+### Min iOS version
+
+Thermion requires a minimum iOS version of 13.0
+
+In your application's `ios/Podfile`, make sure the following line is present (and not commented out):
+
+```ruby
+platform :ios, '13.0'
+```
+