Docs: Doc of tracking add2cart event

[mariorey]

Added documentation about how to track add2cart event

Pull request template

Describe the purpose of the change, the specific changes done in detail, and the issue you have fixed.

Motivation and context

• Dependencies. If any, specify:
• Open issue. If applicable, link: https://searchbroker.atlassian.net/browse/EMP-2699

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that causes existing functionality to not work as expected)
• Change requires a documentation update

What is the destination branch of this PR?

• Main
• Other. Specify:

How has this been tested?

Tests performed according to testing guidelines:

Checklist:

• My code follows the style guidelines of this project.
• I have performed a self-review on my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

[herrardo]

Do not merge, waiting for @mlcorton review. :)

[mlcorton]

Hi there @mariorey! I've noticed that code snippets are breaking numbered lists. I've added a new commit to fix it (fyi, you need to indent code twice to solve it). Also, I've rewritten some content to make it clearer and match the current style and sentence structure. Tell me if I've misled any concept. :)
cc: @herrardo