Docs: Connect Agent and Relay to Providers (#812)

Since the CLI, configuration and much of the features are the same
between Providers, Agents and Relays, adding notes to the Agents and
Relay documentation to point people back to Provider documentation so
they can connect the dots.

docs/src/services/agent.md

@@ -22,6 +22,9 @@ $ quilkin agent --help
 {{#include ../../../target/quilkin.agent.commands}}
 ```
 
+> Each sub-control planes (`file`, `agones`, etc) matches the `quilkin manage` providers capabilities.
+> Have a look at each of the [Control Plane > Providers](../services/xds.md) documentation for integration details.
+
 ## Quickstart
 The simplest version of the `agent` service is just running `quilkin agent`,
 this will setup just the QCMP service allowing the agent to be pinged for

docs/src/services/relay.md

@@ -25,6 +25,8 @@ To view all options for the `relay` subcommand, run:
 $ quilkin relay --help
 {{#include ../../../target/quilkin.relay.commands}}
 ```
+> Each sub-control planes (`file`, `agones`, etc) matches the `quilkin manage` providers capabilities.
+> Have a look at each of the [Control Plane > Providers](../services/xds.md) documentation for integration details.
 
 ## Quickstart
 To get started with the relay service we need to start the relay service, and

docs/src/services/xds.md

@@ -4,7 +4,7 @@
 |----------|-------|---------------------|
 | xDS      | 7800  | gRPC (IPv4 OR IPv6) |
 
-For multi-cluster integration, Quilkin provides a `manage` service, that can be
+For single-cluster integration, Quilkin provides a `manage` service, that can be
 used with a number of configuration discovery providers to provide cluster
 configuration multiple [`proxy`s](./proxy.md). With each provider automating the
 complexity of a full xDS management control plane via integrations with popular