Add docs for Snowplow automatic form tracking

doc/development/snowplow/index.md

@@ -404,6 +404,40 @@ describe('MyTracking', () => {
 });
 ```
 
+### Form tracking
+
+Snowplow automatic [form tracking](https://docs.snowplowanalytics.com/docs/collecting-data/collecting-from-own-applications/javascript-trackers/javascript-tracker/javascript-tracker-v2/tracking-specific-events/#form-tracking) can be enabled by calling `Tracking.enableFormTracking` (after the DOM is ready) and providing a `config` object including at least one of: `forms`, `fields`. The `forms` element determines which forms will be tracked, identified by CSS class name; the `fields` element determines which fields inside the tracked forms will be tracked, identified by field `name`. An optional list of contexts can be provided as the second argument, do note that our [`gitlab_standard`](#gitlab_standard) schema is excluded from these events.
+
+```javascript
+Tracking.enableFormTracking({
+  forms: { allow: ['sign-in-form', 'password-recovery-form'] },
+  fields: { allow: ['terms_and_conditions', 'newsletter_agreement'] },
+});
+```
+
+#### Testing example
+
+```javascript
+import Tracking from '~/tracking';
+
+describe('MyFormTracking', () => {
+  let formTrackingSpy;
+
+  beforeEach(() => {
+    formTrackingSpy = jest
+      .spyOn(Tracking, 'enableFormTracking')
+      .mockImplementation(() => null);
+  });
+
+  it('initialized with the correct configuration', () => {
+    expect(formTrackingSpy).toHaveBeenCalledWith({
+      forms: { allow: ['sign-in-form', 'password-recovery-form'] },
+      fields: { allow: ['terms_and_conditions', 'newsletter_agreement'] },
+    });
+  });
+});
+```
+
 ## Implementing Snowplow Ruby (Backend) tracking
 
 GitLab provides `Gitlab::Tracking`, an interface that wraps the [Snowplow Ruby Tracker](https://github.com/snowplow/snowplow/wiki/ruby-tracker) for tracking custom events.