added aura-ext docs

docs/modules/ROOT/pages/pallets/aura_ext.adoc

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+:source-highlighter: highlight.js
+:highlightjs-languages: rust
+:github-icon: pass:[<svg class="icon"><use href="#github-icon"/></svg>]
+
+= cumulus_aura_ext
+
+Branch/Release: `release-polkadot-v1.3.0`
+
+== Purpose
+
+This pallet integrates parachain’s own block production mechanism (for example AuRa) into Cumulus parachain system. It allows:
+
+- to manage the unincluded blocks from the current slot
+- to validate produced block against the relay chain
+
+== Configuration and Integration link:https://github.com/paritytech/polkadot-sdk/tree/coderobe/release-polkadot-v1.3.0/cumulus/pallets/aura-ext[{github-icon},role=heading-link]
+
+There is no special config for this integration and it has no dispatchables, but you need to integrate it with other `parachain-system` crate:
+
+=== Integrate `BlockExecutor`
+    
+When you invoke the `register_validate_block` macro, you should provide `cumulus_pallet_aura_ext::BlockExecutor` to it to allow `aura-ext` to validate the blocks produced by `aura`
+    
+[source, rust]
+----
+cumulus_pallet_parachain_system::register_validate_block! {
+    Runtime = Runtime,
+    BlockExecutor = cumulus_pallet_aura_ext::BlockExecutor::<Runtime, Executive>,
+}
+----
+    
+=== Integrate `ConsensusHook`
+    
+Also you might want to manage the consensus externally and control the segment that is not yet included (its capacity, speed and etc.) `aura-ext` provides the `FixedVelocityConsensusHook` that allows to check if we are still in the limits for the slot.
+
+[source, rust]
+----
+impl cumulus_pallet_parachain_system::Config for Runtime {
+    ...
+    type ConsensusHook = cumulus_pallet_aura_ext::FixedVelocityConsensusHook<
+        Runtime,
+        RELAY_CHAIN_SLOT_DURATION_MILLIS,
+        BLOCK_PROCESSING_VELOCITY,
+        UNINCLUDED_SEGMENT_CAPACITY,
+    >;
+}
+----