Improve Snowplow Docs and Product Intelligence review guidelines

What does this MR do?

Related to #323735 (closed) and #325774 (closed)

• Add a "Usage recommendations" section
• Add review guidelines for Snowplow, replacing the "Preparing your MR for Review" section
• Improve clarity on the first paragraph of the Frontend implementation
• Improve HAML caveats section
• Reference all existing helpers
• Add more examples
• Improve frontend testing sections
• Add Sisense Discovery examples

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is a docs-only change.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content