add documentation for nightly builds

Author: damdo

docs/book/src/SUMMARY.md

@@ -15,6 +15,7 @@
     - [Flannel](./topics/flannel.md)
 - [Developer Guide](./developers/index.md)
     - [Development](./developers/development.md)
+    - [Try unreleased changes with Nightly Builds](./developers/nightlies.md)
     - [Creating a cluster](./developers/cluster-creation.md)
     - [CI Jobs](./developers/jobs.md)
     - [Releasing](./developers/releasing.md)

docs/book/src/developers/nightlies.md

@@ -0,0 +1,33 @@
+# Nightly Builds
+
+Nightly builds are regular automated builds of the CAPG source code that occur every night.
+
+These builds are generated directly from the latest commit of source code on the main branch.
+
+Nightly builds serve several purposes:
+
+- **Early Testing**: They provide an opportunity for developers and testers to access the most recent changes in the codebase and identify any issues or bugs that may have been introduced.
+- **Feedback Loop**: They facilitate a rapid feedback loop, enabling developers to receive feedback on their changes quickly, allowing them to iterate and improve the code more efficiently.
+- **Preview of New Features**: Users and can get a preview of upcoming features or changes by testing nightly builds, although these builds may not always be stable enough for production use.
+
+Overall, nightly builds play a crucial role in software development by promoting user testing, early bug detection, and rapid iteration.
+
+CAPG Nightly build jobs run in Prow.
+
+## Usage
+
+To try a nightly build, you can download the latest built nightly CAPG manifests, you can find the available ones by executing the following command:
+```bash
+curl -sL -H 'Accept: application/json' "https://storage.googleapis.com/storage/v1/b/k8s-staging-cluster-api-gcp/o" | jq -r '.items | map(select(.name | startswith("components/nightly_main"))) | .[] | [.timeCreated,.mediaLink] | @tsv'
+```
+The output should look something like this:
+```
+2024-05-03T08:03:09.087Z        https://storage.googleapis.com/download/storage/v1/b/k8s-staging-cluster-api-gcp/o/components%2Fnightly_main_2024050x?generation=1714723389033961&alt=media
+2024-05-04T08:02:52.517Z        https://storage.googleapis.com/download/storage/v1/b/k8s-staging-cluster-api-gcp/o/components%2Fnightly_main_2024050y?generation=1714809772486582&alt=media
+2024-05-05T08:02:45.840Z        https://storage.googleapis.com/download/storage/v1/b/k8s-staging-cluster-api-gcp/o/components%2Fnightly_main_2024050z?generation=1714896165803510&alt=media
+```
+
+Now visit the link for the manifest you want to download. This will automatically download the manifest for you.
+
+Once downloaded you can apply the manifest directly to your testing CAPI management cluster/namespace (e.g. with kubectl), as the downloaded CAPG manifest
+will already contain the correct, corresponding CAPG nightly image reference.