Developer documentation for designing feature privileges (#166716)

## Summary

Closes #162263

Introduces a new `Feature Privileges` section to the developer
documentation. The documentation includes a tutorial on how to control
access to features of plugin being developed. Introduces a few sections:

- Controlling access to UI features
- Controlling access to server side APIs
- Documentation for configuration options

## Testing
To build this locally, run ./scripts/dev_docs from a local checkout of
this PR. A server will eventually start on http://localhost:3000 where
you can preview the changes.

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

examples/feature_control_examples/server/plugin.ts

@@ -0,0 +1,90 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+import { CoreSetup, DEFAULT_APP_CATEGORIES, Plugin } from '@kbn/core/server';
+import {
+  PluginSetupContract as FeaturesPluginSetup,
+  // PluginStartContract as FeaturesPluginStart,
+} from '@kbn/features-plugin/server';
+import { FEATURE_PRIVILEGES_PLUGIN_ID } from '../common';
+
+export interface FeatureControlExampleDeps {
+  features: FeaturesPluginSetup;
+}
+
+export class FeatureControlsPluginExample
+  implements Plugin<void, void, any, FeatureControlExampleDeps>
+{
+  public setup(core: CoreSetup, { features }: FeatureControlExampleDeps) {
+    features.registerKibanaFeature({
+      id: FEATURE_PRIVILEGES_PLUGIN_ID,
+      name: 'Feature Plugin Examples',
+      category: DEFAULT_APP_CATEGORIES.management,
+      app: ['FeaturePluginExample'],
+      privileges: {
+        all: {
+          app: ['FeaturePluginExample'],
+          savedObject: {
+            all: [],
+            read: [],
+          },
+          api: ['my_closed_example_api'],
+          ui: ['view', 'create', 'edit', 'delete', 'assign'],
+        },
+        read: {
+          app: ['FeaturePluginExample'],
+          savedObject: {
+            all: [],
+            read: ['tag'],
+          },
+          api: [],
+          ui: ['view'],
+        },
+      },
+    });
+
+    const router = core.http.createRouter();
+    router.get(
+      {
+        path: '/internal/my_plugin/read',
+        validate: false,
+      },
+      async (context, request, response) => {
+        return response.ok({
+          body: {
+            time: new Date().toISOString(),
+          },
+        });
+      }
+    );
+    router.get(
+      {
+        path: '/internal/my_plugin/sensitive_action',
+        validate: false,
+        options: {
+          tags: ['access:my_closed_example_api'],
+        },
+      },
+      async (context, request, response) => {
+        return response.ok({
+          body: {
+            time: new Date().toISOString(),
+          },
+        });
+      }
+    );
+  }
+
+  start() {
+    return {};
+  }
+
+  stop() {
+    return {};
+  }
+}