[ACA-1889] Split Extending section into separate files (#784)

* [ACA-1889] Split Extending section into separate files

* [ACA-1889] Split Extending section into separate files

* [ACA-1889] Added recent changes and then deleted extending.md

* [ACA-1889] Added Tutorials section

docs/extending/creating-custom-evaluators.md

@@ -0,0 +1,68 @@
+---
+---
+
+# Creating custom evaluators
+
+Rule evaluators are plain JavaScript (or TypeScript) functions that take `RuleContext` references and an optional list of `RuleParameter` instances.
+
+Application provides a special [RuleEvaluator](https://github.com/Alfresco/alfresco-content-app/blob/master/src/app/extensions/rule.extensions.ts#L30) type alias for evaluator functions:
+
+```ts
+export type RuleEvaluator = (context: RuleContext, ...args: any[]) => boolean;
+```
+
+Create a function that is going to check if a user has selected one or multiple nodes.
+
+```ts
+export function hasSelection(
+  context: RuleContext,
+  ...args: RuleParameter[]
+): boolean {
+  return !context.selection.isEmpty;
+}
+```
+
+The `context` is a reference to a special instance of the [RuleContext](https://github.com/Alfresco/alfresco-content-app/blob/master/src/app/extensions/rule.extensions.ts#L32) type,
+that provides each evaluator access to runtime entities.
+
+```ts
+export interface RuleContext {
+  selection: SelectionState;
+  navigation: NavigationState;
+  permissions: NodePermissions;
+
+  getEvaluator(key: string): RuleEvaluator;
+}
+```
+
+The `SelectionState` interface exposes information about the global selection state:
+
+```ts
+export interface SelectionState {
+  count: number;
+  nodes: MinimalNodeEntity[];
+  libraries: SiteEntry[];
+  isEmpty: boolean;
+  first?: MinimalNodeEntity;
+  last?: MinimalNodeEntity;
+  folder?: MinimalNodeEntity;
+  file?: MinimalNodeEntity;
+  library?: SiteEntry;
+}
+```
+
+Next, register the function you have created earlier with the `ExtensionService` and give it a unique identifier:
+
+```ts
+extensions.setEvaluators({
+  'plugin1.rules.hasSelection': hasSelection
+});
+```
+
+Now, the `plugin1.rules.hasSelection` evaluator can be used as an inline rule reference,
+or part of the composite rule like `core.every`.
+
+<p class="tip">
+See the [Registration](/extending/registration) section for more details
+on how to register your own entries to be re-used at runtime.
+</p>