Add docs for Snowplow automatic form tracking

What does this MR do?

Related to #329156 (closed)

On !60036 (merged) we added the support for using Snowplow automatic form tracking. This is the corresponding documentation.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)