Improve Snowplow Frontend docs

We need to work on the Implementing Snowplow JS (Frontend) tracking section in our docs.

• Add a "Which tracking method should I use" section before the implementation details
• Add several examples based on usage patterns
• Move the "Tests and test helpers" section right next to the Vue implementation details.
• Remove the Karma test example (related epic: &4900 (closed))
• Add an example of a Raw JS test
• Add an example of a data-attributes test
• Sisense Discovery examples: how a new event will appear, and how the GitLab standard context could be used

/cc @gitlab-org/growth/product-intelligence/engineers