Move all example pallets under examples folder. (#10215)

* Put all examples under one folder

Signed-off-by: Jimmy Chu <jimmychu0807@gmail.com>

* Updated Cargo.toml

Signed-off-by: Jimmy Chu <jimmychu0807@gmail.com>

* updated for ci script

Signed-off-by: Jimmy Chu <jimmychu0807@gmail.com>

* update

Signed-off-by: Jimmy Chu <jimmychu0807@gmail.com>

* Added notes that example pallets are not meant to be used in production.

Signed-off-by: Jimmy Chu <jimmychu0807@gmail.com>

* updated

Signed-off-by: Jimmy Chu <jimmychu0807@gmail.com>

substrate/frame/examples/offchain-worker/README.md

@@ -0,0 +1,29 @@
+<!-- markdown-link-check-disable -->
+# Offchain Worker Example Pallet
+
+The Offchain Worker Example: A simple pallet demonstrating
+concepts, APIs and structures common to most offchain workers.
+
+Run `cargo doc --package pallet-example-offchain-worker --open` to view this module's
+documentation.
+
+- [`pallet_example_offchain_worker::Trait`](./trait.Trait.html)
+- [`Call`](./enum.Call.html)
+- [`Module`](./struct.Module.html)
+
+**This pallet serves as an example showcasing Substrate off-chain worker and is not meant to be
+used in production.**
+
+## Overview
+
+In this example we are going to build a very simplistic, naive and definitely NOT
+production-ready oracle for BTC/USD price.
+Offchain Worker (OCW) will be triggered after every block, fetch the current price
+and prepare either signed or unsigned transaction to feed the result back on chain.
+The on-chain logic will simply aggregate the results and store last `64` values to compute
+the average price.
+Additional logic in OCW is put in place to prevent spamming the network with both signed
+and unsigned transactions, and custom `UnsignedValidator` makes sure that there is only
+one unsigned transaction floating in the network.
+
+License: Unlicense