Add basic overview doc and feature flags

Signed-off-by: killianmuldoon <[email protected]>

Author: killianmuldoon

docs/book/src/SUMMARY.md

@@ -22,6 +22,7 @@
             - [Writing a ClusterClass](./tasks/experimental-features/cluster-class/write-clusterclass.md)
             - [Changing a ClusterClass](./tasks/experimental-features/cluster-class/change-clusterclass.md)
             - [Operating a managed Cluster](./tasks/experimental-features/cluster-class/operate-cluster.md)
+        - [Runtime SDK](./tasks/experimental-features/runtime-sdk.md)
         - [Ignition Bootstrap configuration](./tasks/experimental-features/ignition.md)
 - [Security Guidelines](./security/index.md)
     - [Pod Security Standards](./security/pod-security-standards.md)

docs/book/src/tasks/experimental-features/experimental-features.md

@@ -33,7 +33,9 @@ variables:
   EXP_CLUSTER_RESOURCE_SET: "true"
   EXP_MACHINE_POOL: "true"
   CLUSTER_TOPOLOGY: "true"
+  EXP_RUNTIME_SDK: "true"
 ```
+
 Another way is to set them as environmental variables before running e2e tests.
 
 ## Enabling Experimental Features on Tilt
@@ -45,6 +47,7 @@ kustomize_substitutions:
   EXP_CLUSTER_RESOURCE_SET: 'true'
   EXP_MACHINE_POOL: 'true'
   CLUSTER_TOPOLOGY: 'true'
+  EXP_RUNTIME_SDK: 'true'
 ```
 
 For more details on setting up a development environment with `tilt`, see [Developing Cluster API with Tilt](../../developer/tilt.md)
@@ -73,6 +76,7 @@ Similarly, to **validate** if a particular feature is enabled, see cluster-api-p
 * [ClusterResourceSet](./cluster-resource-set.md)
 * [ClusterClass](./cluster-class/index.md)
 * [Ignition Bootstrap configuration](./ignition.md)
+* [Runtime SDK](./runtime-sdk.md)
 
 **Warning**: Experimental features are unreliable, i.e., some may one day be promoted to the main repository, or they may be modified arbitrarily or even disappear altogether.
 In short, they are not subject to any compatibility or deprecation promise.

docs/book/src/tasks/experimental-features/runtime-sdk.md

@@ -0,0 +1,12 @@
+# Experimental Feature: Runtime SDK
+
+The Runtime SDK feature provides an extensibility mechanism that allows systems, products, and services built on top of Cluster API to hook into a workload cluster’s lifecycle.
+
+
+**Feature gate name**: `RuntimeSDK`
+
+**Variable name to enable/disable the feature gate**: `EXP_RUNTIME_SDK`
+
+
+More details on the Runtime SDK can be found at:
+[RuntimeSDK CAEP](./../../../../proposals/20220221-runtime-SDK.md)