docs: move subscription to tendermint-core (#5323)

## Description

- Move subscriptions to tendermint-core section. 

- Remove unused photot

Closes: #XXX

docs/app-dev/indexing-transactions.md

@@ -4,8 +4,7 @@ order: 6
 
 # Indexing Transactions
 
-Tendermint allows you to index transactions and later query or subscribe
-to their results.
+Tendermint allows you to index transactions and later query or subscribe to their results.
 
 Events can be used to index transactions and blocks according to what happened
 during their execution. Note that the set of events returned for a block from
@@ -78,9 +77,8 @@ func (app *KVStoreApplication) DeliverTx(req types.RequestDeliverTx) types.Resul
 }
 ```
 
-The transaction will be indexed (if the indexer is not `null`) with a certain
-attribute if the attribute's `Index` field is set to `true`. In the above
-example, all attributes will be indexed.
+The transaction will be indexed (if the indexer is not `null`) with a certain attribute if the attribute's `Index` field is set to `true`. 
+In the above example, all attributes will be indexed.
 
 ## Querying Transactions
 

docs/tendermint-core/README.md

@@ -14,6 +14,7 @@ This section dives into the internals of Tendermint the implementation.
 - [Running in Production](./running-in-production.md)
 - [Metrics](./metrics.md)
 - [Validators](./validators.md)
+- [Subscribing to events](./subscription.md)
 - [Block Structure](./block-structure.md)
 - [RPC](./rpc.md)
 - [Fast Sync](./fast-sync.md)

docs/tendermint-core/block-structure.md

@@ -1,5 +1,5 @@
 ---
-order: 7
+order: 8
 ---
 
 # Block Structure

docs/tendermint-core/fast-sync.md

@@ -1,5 +1,5 @@
 ---
-order: 9
+order: 10
 ---
 
 # Fast Sync

docs/tendermint-core/light-client.md

@@ -1,5 +1,5 @@
 ---
-order: 12
+order: 13
 ---
 
 # Light Client

docs/tendermint-core/mempool.md

@@ -1,5 +1,5 @@
 ---
-order: 11
+order: 12
 ---
 
 # Mempool

docs/tendermint-core/rpc.md

@@ -1,5 +1,5 @@
 ---
-order: 8
+order: 9
 ---
 
 # RPC

docs/tendermint-core/state-sync.md

@@ -1,5 +1,5 @@
 --- 
-order: 10
+order: 11
 ---
 
 # State Sync

docs/tendermint-core/subscription.md

@@ -1,5 +1,5 @@
 ---
-order: 5
+order: 7
 ---
 
 # Subscribing to events via Websocket