feat: add interceptor page

Closes: #18

docs/docs/interceptors.md

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+# Interceptors
+
+An interceptor is a class annotated with the @@Interceptor@@ decorator. Interceptors should implement the @@InterceptorMethods@@ interface.
+
+Interceptors have a set of useful capabilities which are inspired by the [Aspect Oriented Programming](https://en.wikipedia.org/wiki/Aspect-oriented_programming) (AOP) technique.
+
+Creating and consuming an interceptor is a two-step process:
+
+1. Create and annotate the interceptor class that will intercept calls to service methods
+2. Decide which methods will be **intercepted** by which **interceptor**
+
+They make it possible to:
+
+- bind extra logic before/after method execution
+- transform the result returned from a function
+- transform the exception thrown from a function
+- extend the basic function's behavior
+- completely override a function depending on specific conditions
+
+## Decorators
+
+<ApiList query="module == '@tsed/di' && symbolType === 'decorator'" />
+
+## Interceptor class
+
+To create an interceptor class you need to implement the @@InterceptorMethods@@ interface and implement the
+`intercept(context: InterceptorContext<any>, next?: InterceptorNext)` method, and use the `@Interceptor()` annotation to register your interceptor class.
+
+Inside your `src/interceptors/MyInterceptor.ts` file, create the following simple interceptor.
+
+<<< @/docs/snippets/interceptors/interceptor-example.ts
+
+## Use the interceptor
+
+Now that your interceptor logic is in place, you can use it in any other service. You need to use the `@Intercept(InterceptorClass, opts)` annotation to register which interceptor should be used for the specific method you want to intercept. 
+An example service in `src/services/MyService.ts`:
+
+<<< @/docs/snippets/interceptors/interceptor-usage.ts
+
+If the service method is executed like `myServiceInstance.mySimpleMethod()` we will get the following output:
+
+```bash
+the method mySimpleMethod will be executed with args and static data simple data
+the simple method is executed
+the method was executed, and returned undefined
+```
+
+## Catch error with Interceptor
+
+You can also catch errors with an interceptor. 
+To do this, you need to implement the `intercept` method in your interceptor class:
+
+<<< @/docs/snippets/interceptors/interceptor-catch-error.ts

docs/docs/snippets/interceptors/interceptor-example.ts

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+import {InterceptorMethods, InterceptorContext, InterceptorNext, Interceptor} from "@tsed/di";
+
+@Interceptor()
+export class MyInterceptor implements InterceptorMethods {
+  /**
+   * ctx: The context that holds the dynamic data related to the method execution and the proceed method
+   * to proceed with the original method execution
+   *
+   * opts: Static params that can be provided when the interceptor is attached to a specific method
+   */
+  async intercept(context: InterceptorContext<any>, next: InterceptorNext) {
+    console.log(`the method ${context.propertyKey} will be executed with args ${context.args} and static data ${context.options}`);
+    // let the original method by calling next function
+    const result = await next();
+
+    console.log(`the method was executed, and returned ${result}`);
+
+    // must return the returned value back to the caller
+    return result;
+  }
+}

docs/docs/snippets/interceptors/interceptor-usage.ts

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+import {Intercept, Injectable} from "@tsed/di";
+import {MyInterceptor} from "../interceptors/MyInterceptor";
+
+@Injectable()
+export class MyService {
+  // MyInterceptor is going to be used to intercept this method whenever called
+  // 'simple data' is static data that will be passed as second arg to the interceptor.intercept method
+  @Intercept(MyInterceptor, "simple data")
+  mySimpleMethod() {
+    console.log("the simple method is executed");
+  }
+}
+
+// on class
+@Injectable()
+@Intercept(MyInterceptor, "simple data")
+export class MyService2 {
+  mySimpleMethod1() {
+    console.log("the simple method is executed");
+  }
+
+  mySimpleMethod2() {
+    console.log("the simple method is executed");
+  }
+}