build(aio): improve the API Pipe pages (#22702)

This change adds:

* an impure badge for Pipes that are marked as  `pure: false`
* a pipe specific overview that shows the syntax for using a pipe in a template.
* an "input value" section describing the type of the value that the pipe expects.
* a "pipe params" section describing any additional params that a pipe expects.

PR Close #22702

aio/tests/e2e/api.e2e-spec.ts

@@ -51,6 +51,12 @@ describe('Api pages', function() {
     expect(page.getSection('instance-properties').isPresent()).toBe(false);
   });
 
+  it('should show "impure" badge if pipe is not pure', () => {
+    const page = new ApiPage('api/common/AsyncPipe');
+    const impureBadge = page.getBadge('impure-pipe');
+    expect(impureBadge.isPresent()).toBe(true);
+  });
+
   it('should show links to github', () => {
     const page = new ApiPage('api/core/EventEmitter');
     /* tslint:disable:max-line-length */

aio/tests/e2e/api.po.ts

@@ -35,4 +35,8 @@ export class ApiPage extends SitePage {
   getSection(cls) {
     return element(by.css(`section.${cls}`));
   }
+
+  getBadge(cls) {
+    return element(by.css('.api-status-label.' +  cls));
+  }
 }