docs: add more docs

docs/guide/src/async/fetching.md

@@ -1 +1,4 @@
 # Fetching
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+This section is currently under construction! 🏗

docs/guide/src/async/index.md

@@ -17,3 +17,6 @@ Writing apps that deal with Send/Sync can be frustrating at times. Under the hoo
 
 
 All async code in your app is polled on a `LocalSet`, so any async code we w
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+> This section is currently under construction! 🏗