docs: add package godoc for indexer (#6839)

* docs: add indexer godoc

* docs++

* docs++

* docs++

* docs++

* docs++

* Update state/indexer/doc.go

Co-authored-by: M. J. Fromberger <fromberger@interchain.io>

* Update state/indexer/doc.go

Co-authored-by: M. J. Fromberger <fromberger@interchain.io>

* Update state/indexer/doc.go

Co-authored-by: M. J. Fromberger <fromberger@interchain.io>

* Update state/indexer/doc.go

Co-authored-by: M. J. Fromberger <fromberger@interchain.io>

* Update state/indexer/doc.go

Co-authored-by: M. J. Fromberger <fromberger@interchain.io>

* Update state/indexer/doc.go

Co-authored-by: M. J. Fromberger <fromberger@interchain.io>

* docs++

Co-authored-by: M. J. Fromberger <fromberger@interchain.io>

state/indexer/doc.go

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+/*
+Package indexer defines Tendermint's block and transaction event indexing logic.
+
+Tendermint supports two primary means of block and transaction event indexing:
+
+1. A key-value sink via an embedded database with a proprietary query language.
+2. A Postgres-based sink.
+
+An ABCI application can emit events during block and transaction execution in the form
+
+   <abci.Event.Type>.<abci.EventAttributeKey>=<abci.EventAttributeValue>
+
+for example "transfer.amount=10000".
+
+An operator can enable one or both of the supported indexing sinks via the
+'tx-index.indexer' Tendermint configuration.
+
+Example:
+
+	[tx-index]
+	indexer = ["kv", "psql"]
+
+If an operator wants to completely disable indexing, they may simply just provide
+the "null" sink option in the configuration. All other sinks will be ignored if
+"null" is provided.
+
+If indexing is enabled, the indexer.Service will iterate over all enabled sinks
+and invoke block and transaction indexing via the appropriate IndexBlockEvents
+and IndexTxEvents methods.
+
+Note, the "kv" sink is considered deprecated and its query functionality is very
+limited, but does allow users to directly query for block and transaction events
+against Tendermint's RPC. Instead, operators are encouraged to use the "psql"
+indexing sink when more complex queries are required and for reliability purposes
+as PostgreSQL can scale.
+
+Prior to starting Tendermint with the "psql" indexing sink enabled, operators
+must ensure the following:
+
+1. The "psql" indexing sink is provided in Tendermint's configuration.
+2. A 'tx-index.psql-conn' value is provided that contains the PostgreSQL connection URI.
+3. The block and transaction event schemas have been created in the PostgreSQL database.
+
+Tendermint provides the block and transaction event schemas in the following
+path: state/indexer/sink/psql/schema.sql
+
+To create the schema in a PostgreSQL database, perform the schema query
+manually or invoke schema creation via the CLI:
+
+	$ psql <flags> -f state/indexer/sink/psql/schema.sql
+
+The "psql" indexing sink prohibits queries via RPC. When using a PostgreSQL sink,
+queries can and should be made directly against the database using SQL.
+
+The following are some example SQL queries against the database schema:
+
+* Query for all transaction events for a given transaction hash:
+
+	SELECT * FROM tx_events WHERE hash = '3E7D1F...';
+
+* Query for all transaction events for a given block height:
+
+	SELECT * FROM tx_events WHERE height = 25;
+
+* Query for transaction events that have a given type (i.e. value wildcard):
+
+	SELECT * FROM tx_events WHERE key LIKE '%transfer.recipient%';
+
+Note that if a complete abci.TxResult is needed, you will need to join "tx_events" with
+"tx_results" via a foreign key, to obtain contains the raw protobuf-encoded abci.TxResult.
+*/
+package indexer