Document `UI Event Transaction`

[bruno-garcia]

This feature was introduced to some Mobile SDKs. It's also known as _User Interaction Tracking".

The goal is for the SDK to create a transaction when the user interacts with the app. These are based on 'idle timeout' since there's no clear indication of when the transaction ended.

The general tracking issue for it:

• UI Event Transactions - iOS and Android team-mobile#4

Docs:

• Android docs
• iOS docs

We need to write the guideline for SDK engineers. Some of the operations used are already documented, and we could link to it: https://develop.sentry.dev/sdk/performance/span-operations/#mobile