Add doc for DHT bootnodes mechanism (#8865)

Add description of the RFC-8 "DHT bootnodes" implementation.

Rendered:
https://github.com/paritytech/polkadot-sdk/blob/dm-dht-bootnodes-doc/docs/sdk/src/guides/dht_bootnodes.md

Author: dmitry-markin

docs/sdk/src/guides/dht_bootnodes.md

@@ -0,0 +1,35 @@
+# DHT Bootnodes (RFC-8)
+
+The "DHT bootnodes" mechanism, as defined in [RFC-0008: Store parachain bootnodes in relay chain
+DHT](https://polkadot-fellows.github.io/RFCs/approved/0008-parachain-bootnodes-dht.html)
+and implemented in Polkadot, enables parachain nodes to bootstrap without requiring hardcoded
+bootnode addresses in the chainspec.
+
+## How It Works
+
+This mechanism, enabled by default, allows any parachain node to serve as a bootnode. In each
+epoch, 20 parachain nodes are selected as bootnodes based on the proximity of their relay chain
+peer IDs to the parachain key for that epoch. These selected nodes register themselves in the relay
+chain's Kademlia DHT as [_content providers._](
+https://github.com/libp2p/specs/tree/master/kad-dht#content-provider-advertisement-and-discovery)
+Other nodes can then discover and query them to obtain the multiaddresses of their parachain
+instances.
+
+## Information for Parachain Operators
+
+The DHT bootnode mechanism simplifies parachain deployment by removing the need for dedicated
+bootnodes and hardcoded addresses in the chainspec. It also reduces the risk of single points
+of failure if predefined bootnodes become unreachable.
+
+However, since this feature is relatively new, high-value parachains are still advised to include
+a set of dedicated bootnodes in the chainspec as a fallback mechanism. Also, the bootnodes
+specified via the `--bootnodes` command-line option are always used.
+
+## Command-Line Options
+
+There are two independent CLI options controlling the mechanism:
+
+- `--no-dht-bootnode` prevents a node from acting as a DHT bootnode.
+- `--no-dht-bootnode-discovery` disables discovery of other parachain nodes via the DHT bootnode
+  mechanism.
+