Documentation: Adding section for Projections to the Modules summary… (#30175)

* Documentation:  Adding section for Projections to the Modules summary page #29710

* Added Projections entry in the module summary index.
* Added Projections section with brief description and dependency reference.

* Documentation:  Cross-links between core modules guide and other modules page  #29710

* Removed HTTP section from core modules page (redundant with section in other modules page)
* Added link to other modules page on the core modules guide
* Added link to the core modules guide from the other modules page.

* Documentation:  Adding sections for Alpakka and gRPC to modules guide.  Adding original HTTP section back in as well. #29710

akka-docs/src/main/paradox/common/other-modules.md

@@ -1,5 +1,7 @@
 # Other Akka modules
 
+This page describes modules that compliment libraries from the Akka core.  See [this overview](https://doc.akka.io/docs/akka/current/typed/guide/modules.html) instead for a guide on the core modules.
+
 ## [Akka HTTP](https://doc.akka.io/docs/akka-http/current/)
 
 A full server- and client-side HTTP stack on top of akka-actor and akka-stream.

akka-docs/src/main/paradox/typed/guide/modules.md

@@ -10,9 +10,13 @@ The following capabilities are included with Akka OSS and are introduced later o
 * @ref:[Cluster Sharding](#cluster-sharding)
 * @ref:[Cluster Singleton](#cluster-singleton)
 * @ref:[Persistence](#persistence)
+* @ref:[Projections](#projections)
 * @ref:[Distributed Data](#distributed-data)
 * @ref:[Streams](#streams)
+* @ref:[Alpakka](#alpakka)
 * @ref:[HTTP](#http)
+* @ref:[gRPC](#grpc)
+* [Other Akka modules](https://doc.akka.io/docs/akka/current/common/other-modules.html)
 
 With a [Lightbend Platform Subscription](https://www.lightbend.com/lightbend-subscription), you can use [Akka Enhancements](https://doc.akka.io/docs/akka-enhancements/current/) that includes:
 
@@ -180,6 +184,26 @@ Persistence tackles the following challenges:
 * How to introspect domain events that have led an entity to its current state.
 * How to leverage [Event Sourcing](https://martinfowler.com/eaaDev/EventSourcing.html) in your application to support long-running processes while the project continues to evolve.
 
+### Projections
+
+@@dependency[sbt,Maven,Gradle] {
+  bomGroup=com.typesafe.akka bomArtifact=akka-bom_$scala.binary.version$ bomVersionSymbols=AkkaVersion
+  symbol1=AkkaVersion
+  value1="$akka.version$"
+  group=com.typesafe.akka
+  artifact=akka-projection-core_$scala.binary.version$
+  version=AkkaVersion
+}
+
+Projections provides a simple API for consuming a stream of events for projection into a variety of downstream options.  The core dependency provides only the API and other provider dependencies are required for different source and sink implementations.
+
+Challenges Projections solve include the following:
+
+* Constructing alternate or aggregate views over an event stream.
+* Propagating an event stream onto another downstream medium such as a Kafka topic.  
+* A simple way of building read-side projections in the context of [Event Sourcing](https://martinfowler.com/eaaDev/EventSourcing.html) and [CQRS system](https://docs.microsoft.com/en-us/previous-versions/msp-n-p/jj591573%28v=pandp.10%29)
+
+
 ### Distributed Data
 
 @@dependency[sbt,Maven,Gradle] {
@@ -229,6 +253,19 @@ Streams solve the following challenges:
 * How to connect asynchronous services in a flexible way to each other with high performance.
 * How to provide or consume Reactive Streams compliant interfaces to interface with a third party library.
 
+### Alpakka
+
+[Alpakka](https://doc.akka.io/docs/alpakka/current/) is a separate module from Akka.
+
+Alpakka is collection of modules built upon the Streams API to provide Reactive Stream connector
+implementations for a variety of technologies common in the cloud and infrastructure landscape.  
+See the [Alpakka overview page](https://doc.akka.io/docs/alpakka/current/overview.html) for more details on the API and the implementation modules available.
+
+Alpakka helps solve the following challenges:
+
+* Connecting various infrastructure or persistence components to Stream based flows.
+* Connecting to legacy systems in a manner that adheres to a Reactive Streams API.
+
 ### HTTP
 
 [Akka HTTP](https://doc.akka.io/docs/akka-http/current/) is a separate module from Akka.
@@ -242,6 +279,22 @@ Some of the challenges that HTTP tackles:
 * How to stream large datasets in and out of a system using HTTP.
 * How to stream live events in and out of a system using HTTP.
 
+### gRPC
+
+[Akka gRPC](https://doc.akka.io/docs/akka-grpc/current/index.html) is a separate module from Akka.
+
+This library provides an implementation of gRPC that integrates nicely with the @ref:[HTTP](#http) and @ref:[Streams](#streams) modules.  It is capable of generating both client and server-side artifacts from protobuf service definitions, which can then be exposed using Akka HTTP, and handled using Streams.
+
+Some of the challenges that Akka gRPC tackles:
+
+* Exposing services with all the benefits of gRPC & protobuf:  
+  * Schema-first contract
+  * Schema evolution support
+  * Efficient binary protocol
+  * First-class streaming support
+  * Wide interoperability
+  * Use of HTTP/2 connection multiplexing
+
 ### Example of module use
 
 Akka modules integrate together seamlessly. For example, think of a large set of stateful business objects, such as documents or shopping carts, that website users access. If you model these as sharded entities, using Sharding and Persistence, they will be balanced across a cluster that you can scale out on-demand. They will be available during spikes that come from advertising campaigns or before holidays will be handled, even if some systems crash. You can also take the real-time stream of domain events with Persistence Query and use Streams to pipe them into a streaming Fast Data engine. Then, take the output of that engine as a Stream, manipulate it using Akka Streams