Merge branch 'kassio/gitlab-pages-docs-about-feature-flags' into 'master'

Add documentation about GitLab Pages feature flags

See merge request gitlab-org/gitlab!93287

doc/development/feature_flags/index.md

@@ -178,6 +178,9 @@ All validations are skipped when running in `RAILS_ENV=production`.
 
 ## Create a new feature flag
 
+NOTE:
+GitLab Pages uses [a different process](../pages/index.md#feature-flags) for feature flags.
+
 The GitLab codebase provides [`bin/feature-flag`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/bin/feature-flag),
 a dedicated tool to create new feature flag definitions.
 The tool asks various questions about the new feature flag, then creates

doc/development/pages/index.md

@@ -236,3 +236,29 @@ make acceptance
 # so we want to have the latest changes in the build that is tested
 make && go test ./ -run TestRedirect
 ```
+
+## Contributing
+
+### Feature flags
+
+WARNING:
+All newly-introduced feature flags should be [disabled by default](https://about.gitlab.com/handbook/product-development-flow/feature-flag-lifecycle/#feature-flags-in-gitlab-development).
+
+Consider adding a [feature flag](../feature_flags/index.md) for any non-trivial changes.
+Feature flags can make the release and rollback of these changes easier, avoiding
+incidents and downtime. To add a new feature flag to GitLab Pages:
+
+1. Create the feature flag in
+   [`internal/feature/feature.go`](https://gitlab.com/gitlab-org/gitlab-pages/-/blob/master/internal/feature/feature.go),
+   which must be **off** by default.
+1. Create an issue to track the feature flag using the `Feature Flag` template.
+1. Add the `~"feature flag"` label to any merge requests that handle feature flags.
+
+For GitLab Pages, the feature flags are controlled by environment variables at a global level. It
+A deployment at the service level is required to change the state of a feature flag.
+Example of an merge request enabling a GitLab Pages feature flag:
+[Enforce GitLab Pages rate limits](https://gitlab.com/gitlab-com/gl-infra/k8s-workloads/gitlab-com/-/merge_requests/1500)
+
+## Related topics
+
+- [Feature flags in the development of GitLab](../feature_flags/index.md)