Create a mixin that contains all the fields that Alloy generates automatically

In the current documentation for getting started with the AEP Web SDK (Alloy) library, there are instructions to create a schema. The user is instructed to create a new schema and add 3 mixins. We'd like to be able to have one mixin specifically for the AEP Web SDK to make this step easier for users to follow. [(see current documentation)](https://www.adobe.io/apis/experienceplatform/home/services/web-sdk.html#!api-specification/markdown/narrative/edge/getting-started/quick-start-with-launch.md)

## What are the schemas that are affected by the issue
The PR for this issue will create a new ExperienceEvent mixin.

## What are examples of products that are impacted by the issue
AEP Web SDK (Alloy)

Start with the current AEP Web SDK (Alloy) getting-started documentation linked in the issue and review the schema setup that currently uses three mixins. Then inspect the repository's ExperienceEvent schema and related mixins to determine the fields Alloy generates automatically. Done means a new ExperienceEvent mixin contains those fields and supports the documented setup described in the issue.

Written by the indexing model from the issue text.