Add developer guide for verifying GA events and network requests.

Currently, debugging Google Analytics events in the Oppia frontend requires contributors to manually inspect:

window.dataLayer

No network requests are sent to https://www.google-analytics.com/, which makes it harder to verify whether an event was actually registered or not.

Also, having access to a personal GA dashboard while debugging locally would make it much easier to confirm that events are firing correctly.

It would be helpful to add documentation explaining how contributors can:

• set up their own Google Analytics account
• connect it to Oppia’s dev mode
• verify analytics events locally

Without this, debugging analytics-related issues can be confusing and will require multiple rounds of testing on backup and other servers to confirm the cause and solution.