refactor(baseapp): create checktx handler (backport #21979) (#22044)

Co-authored-by: Marko <marko@baricevic.me>

docs/build/abci/04-checktx.md

@@ -0,0 +1,50 @@
+# CheckTx
+
+CheckTx is called by the `BaseApp` when comet receives a transaction from a client, over the p2p network or RPC. The CheckTx method is responsible for validating the transaction and returning an error if the transaction is invalid. 
+
+```mermaid
+graph TD
+    subgraph SDK[Cosmos SDK]
+        B[Baseapp]
+        A[AnteHandlers]
+        B <-->|Validate TX| A
+    end
+    C[CometBFT] <-->|CheckTx|SDK
+    U((User)) -->|Submit TX| C
+    N[P2P] -->|Receive TX| C
+```
+
+```go reference
+https://github.com/cosmos/cosmos-sdk/blob/31c604762a434c7b676b6a89897ecbd7c4653a23/baseapp/abci.go#L350-L390
+```
+
+## CheckTx Handler
+
+`CheckTxHandler` allows users to extend the logic of `CheckTx`. `CheckTxHandler` is called by pasding context and the transaction bytes received through ABCI. It is required that the handler returns deterministic results given the same transaction bytes. 
+
+:::note
+we return the raw decoded transaction here to avoid decoding it twice.
+:::
+
+```go
+type CheckTxHandler func(ctx sdk.Context, tx []byte) (Tx, error)
+```
+
+Setting a custom `CheckTxHandler` is optional. It can be done from your app.go file:
+
+```go
+func NewSimApp(
+	logger log.Logger,
+	db corestore.KVStoreWithBatch,
+	traceStore io.Writer,
+	loadLatest bool,
+	appOpts servertypes.AppOptions,
+	baseAppOptions ...func(*baseapp.BaseApp),
+) *SimApp {
+  ...
+  // Create ChecktxHandler
+  checktxHandler := abci.NewCustomCheckTxHandler(...)
+  app.SetCheckTxHandler(checktxHandler)
+  ...
+}
+```

docs/build/building-apps/02-app-mempool.md

@@ -22,7 +22,7 @@ Notably it introduces the `PrepareProposal` and `ProcessProposal` steps of ABCI+
 
 ## Mempool
 
-+ Before we delve into `PrepareProposal` and `ProcessProposal`, let's first walk through the mempool concepts.
+* Before we delve into `PrepareProposal` and `ProcessProposal`, let's first walk through the mempool concepts.
 
 There are countless designs that an application developer can write for a mempool, the SDK opted to provide only simple mempool implementations.
 Namely, the SDK provides the following mempools: