docs: Add gRPC block documentation for loki.source.awsfirehose

The loki.source.awsfirehose component uses the shared ServerConfig
(internal/component/common/net/config.go), which always binds both
HTTP and gRPC listeners regardless of whether the blocks are
configured. The docs only referenced the http block, leaving the
gRPC port undocumented -- the same root cause addressed for
loki.source.api in #5919.

Add the grpc and grpc > tls rows to the Blocks table, add the
grpc reference, and include the shared loki-server-grpc.md file,
matching the pattern used by loki.source.api, loki.source.heroku,
and loki.source.gcplog.

Author: boinger

docs/sources/reference/components/loki/loki.source.awsfirehose.md

@@ -107,14 +107,21 @@ You can use the following blocks with `loki.source.awsfirehose`:
 
 | Name                  | Description                                        | Required |
 | --------------------- | -------------------------------------------------- | -------- |
+| [`grpc`][grpc]        | Configures the gRPC server that receives requests. | no       |
+| `grpc` > [`tls`][tls] | Configures TLS for the gRPC server.                | no       |
 | [`http`][http]        | Configures the HTTP server that receives requests. | no       |
 | `http` > [`tls`][tls] | Configures TLS for the HTTP server.                | no       |
 
 [http]: #http
+[grpc]: #grpc
 [tls]: #tls
 
 {{< /docs/alloy-config >}}
 
+### `grpc`
+
+{{< docs/shared lookup="reference/components/loki-server-grpc.md" source="alloy" version="<ALLOY_VERSION>" >}}
+
 ### `http`
 
 {{< docs/shared lookup="reference/components/server-http.md" source="alloy" version="<ALLOY_VERSION>" >}}